Looks like an AppArmor issue. Here's an excerpt from dmesg when I tried this myself (Ubuntu 14.04.2 LTS):

[  303.492810] audit_printk_skb: 9 callbacks suppressed
[  303.492816] type=1400 audit(1425023716.919:96): apparmor="DENIED" operation="connect" profile="/usr/lib/lightdm/lightdm-guest-session" name="/tmp/.X11-unix/X1" pid=3318 comm="xrdb" requested_mask="rw" denied_mask="rw" fsuid=121 ouid=0
[  303.729035] type=1400 audit(1425023717.154:97): apparmor="DENIED" operation="connect" profile="/usr/lib/lightdm/lightdm-guest-session" name="/tmp/.X11-unix/X1" pid=3331 comm="xhost" requested_mask="rw" denied_mask="rw" fsuid=121 ouid=0
[  304.168695] type=1400 audit(1425023717.594:98): apparmor="DENIED" operation="connect" profile="/usr/lib/lightdm/lightdm-guest-session" name="/tmp/.X11-unix/X1" pid=3346 comm="xhost" requested_mask="rw" denied_mask="rw" fsuid=121 ouid=0
[  305.495341] type=1400 audit(1425023718.918:99): apparmor="DENIED" operation="connect" profile="/usr/lib/lightdm/lightdm-guest-session" name="/tmp/.X11-unix/X1" pid=3401 comm="VBoxClient" requested_mask="rw" denied_mask="rw" fsuid=121 ouid=0
[  305.612833] type=1400 audit(1425023719.038:100): apparmor="DENIED" operation="connect" profile="/usr/lib/lightdm/lightdm-guest-session" name="/tmp/.X11-unix/X1" pid=3409 comm="VBoxClient" requested_mask="rw" denied_mask="rw" fsuid=121 ouid=0
[  305.674717] type=1400 audit(1425023719.097:101): apparmor="DENIED" operation="connect" profile="/usr/lib/lightdm/lightdm-guest-session" name="/tmp/.X11-unix/X1" pid=3413 comm="VBoxClient" requested_mask="rw" denied_mask="rw" fsuid=121 ouid=0
[  305.755117] type=1400 audit(1425023719.177:102): apparmor="DENIED" operation="connect" profile="/usr/lib/lightdm/lightdm-guest-session" name="/tmp/.X11-unix/X1" pid=3417 comm="VBoxClient" requested_mask="rw" denied_mask="rw" fsuid=121 ouid=0
[  305.806479] type=1400 audit(1425023719.229:103): apparmor="DENIED" operation="mount" info="failed mntpnt match" error=-13 profile="/usr/lib/lightdm/lightdm-guest-session" name="/proc/" pid=3312 comm="init" fstype="proc" srcname="proc" flags="rw"
[  305.806646] type=1400 audit(1425023719.229:104): apparmor="DENIED" operation="mount" info="failed mntpnt match" error=-13 profile="/usr/lib/lightdm/lightdm-guest-session" name="/sys/" pid=3312 comm="init" fstype="sysfs" srcname="sysfs" flags="rw"
[  306.856473] type=1400 audit(1425023720.281:105): apparmor="DENIED" operation="open" profile="/usr/lib/lightdm/lightdm-guest-session" name="/proc/3442/cmdline" pid=3427 comm="dbus-daemon" requested_mask="r" denied_mask="r" fsuid=121 ouid=0
[  310.443658] audit_printk_skb: 9 callbacks suppressed
[  310.443678] type=1400 audit(1425023723.863:109): apparmor="DENIED" operation="mount" info="failed mntpnt match" error=-13 profile="/usr/lib/lightdm/lightdm-guest-session" name="/run/user/121/gvfs/" pid=3521 comm="gvfsd-fuse" fstype="fuse.gvfsd-fuse" srcname="gvfsd-fuse" flags="rw, nosuid, nodev"
[  310.875218] type=1400 audit(1425023724.295:110): apparmor="DENIED" operation="connect" profile="/usr/lib/lightdm/lightdm-guest-session" name="/tmp/.X11-unix/X1" pid=3519 comm="ibus-ui-gtk3" requested_mask="rw" denied_mask="rw" fsuid=121 ouid=0
[  310.961406] type=1400 audit(1425023724.387:111): apparmor="DENIED" operation="connect" profile="/usr/lib/lightdm/lightdm-guest-session" name="/tmp/.X11-unix/X1" pid=3522 comm="ibus-x11" requested_mask="rw" denied_mask="rw" fsuid=121 ouid=0
[  311.180376] type=1400 audit(1425023724.607:112): apparmor="DENIED" operation="connect" profile="/usr/lib/lightdm/lightdm-guest-session" name="/tmp/.X11-unix/X1" pid=3529 comm="at-spi-bus-laun" requested_mask="rw" denied_mask="rw" fsuid=121 ouid=0
[  311.375818] type=1400 audit(1425023724.799:113): apparmor="DENIED" operation="connect" profile="/usr/lib/lightdm/lightdm-guest-session" name="/tmp/.X11-unix/X1" pid=3529 comm="at-spi-bus-laun" requested_mask="rw" denied_mask="rw" fsuid=121 ouid=0
[  311.391770] type=1400 audit(1425023724.815:114): apparmor="DENIED" operation="connect" profile="/usr/lib/lightdm/lightdm-guest-session" name="/tmp/.X11-unix/X1" pid=3519 comm="ibus-ui-gtk3" requested_mask="rw" denied_mask="rw" fsuid=121 ouid=0
[  311.589301] type=1400 audit(1425023725.015:115): apparmor="DENIED" operation="connect" profile="/usr/lib/lightdm/lightdm-guest-session" name="/tmp/.X11-unix/X1" pid=3540 comm="at-spi2-registr" requested_mask="rw" denied_mask="rw" fsuid=121 ouid=0
[  311.633281] type=1400 audit(1425023725.058:116): apparmor="DENIED" operation="connect" profile="/usr/lib/lightdm/lightdm-guest-session" name="/tmp/.X11-unix/X1" pid=3540 comm="at-spi2-registr" requested_mask="rw" denied_mask="rw" fsuid=121 ouid=0
[  312.810857] type=1400 audit(1425023726.234:117): apparmor="DENIED" operation="connect" profile="/usr/lib/lightdm/lightdm-guest-session" name="/tmp/.X11-unix/X1" pid=3555 comm="lxsession" requested_mask="rw" denied_mask="rw" fsuid=121 ouid=0
[  312.943315] type=1400 audit(1425023726.366:118): apparmor="DENIED" operation="connect" profile="/usr/lib/lightdm/lightdm-guest-session" name="/tmp/.X11-unix/X1" pid=3529 comm="at-spi-bus-laun" requested_mask="rw" denied_mask="rw" fsuid=121 ouid=0

Renaming /etc/apparmor.d/lightdm-guest-session to lightdm-guest-session~ allows successful guest login for me.