Answer to 1st question 1st;
 I've got this on both a saucy and a trusty guest, so not new in Trusty, but Raring doesn't seem to exhibit it

Linux saucy 3.12.0-031200-generic #201311031935 SMP Mon Nov 4 00:36:54 UTC 2013 x86_64 x86_64 x86_64 GNU/Linux
still oopsing - slightly different backtrace and it took three vt changes to trigger it:

[   66.200441] Kernel BUG at ffffffffa01114e0 [verbose debug info unavailable]
[   66.200445] invalid opcode: 0000 [#1] SMP 
[   66.200457] Modules linked in: bnep rfcomm bluetooth qxl ttm drm_kms_helper ppdev drm psmouse i2c_piix4 serio_raw virtio_balloon virtio_console lp microcode mac_hid parport_pc parport floppy
[   66.200462] CPU: 0 PID: 1033 Comm: Xorg Not tainted 3.12.0-031200-generic #201311031935
[   66.200464] Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
[   66.200465] task: ffff880078c02f60 ti: ffff880078cd2000 task.ti: ffff880078cd2000
[   66.200475] RIP: 0010:[<ffffffffa01114e0>]  [<ffffffffa01114e0>] qxl_send_monitors_config+0x150/0x160 [qxl]
[   66.200477] RSP: 0018:ffff880078cd3698  EFLAGS: 00010246
[   66.200478] RAX: ffffc900003d8000 RBX: ffff88007a5c0000 RCX: ffffffffa011aeb8
[   66.200480] RDX: ffffffffa011a640 RSI: ffffffffa011b769 RDI: ffff88007a5c0000
[   66.200481] RBP: ffff880078cd36b8 R08: 0000000000000000 R09: 0000000000000000
[   66.200483] R10: 0000000000000001 R11: 0000000000000000 R12: ffffc900003d2004
[   66.200484] R13: ffff88007986fd68 R14: ffff88007c39e000 R15: ffff8800795af700
[   66.200487] FS:  00007fb411855980(0000) GS:ffff88007fc00000(0000) knlGS:0000000000000000
[   66.200489] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   66.200490] CR2: 00007fa4549980e0 CR3: 000000007957a000 CR4: 00000000000006f0
[   66.200502] Stack:
[   66.200506]  ffff880079863000 ffff88007c39e420 0000000000000000 ffff88007a5c0000
[   66.200509]  ffff880078cd3718 ffffffffa01117f0 0000030000000000 0000000000000400
[   66.200512]  ffff880000000300 ffff880000000001 0000000000000000 ffff88007a5c0000
[   66.200513] Call Trace:
[   66.200521]  [<ffffffffa01117f0>] qxl_write_monitors_config_for_encoder+0x130/0x210 [qxl]
[   66.200527]  [<ffffffffa01118eb>] qxl_enc_commit+0x1b/0x40 [qxl]
[   66.200534]  [<ffffffffa003c4c1>] drm_crtc_helper_set_mode+0x431/0x5c0 [drm_kms_helper]
[   66.200542]  [<ffffffffa003d74a>] drm_crtc_helper_set_config+0x8fa/0xb70 [drm_kms_helper]
[   66.200561]  [<ffffffffa00ae5cc>] drm_mode_set_config_internal+0x5c/0xe0 [drm]
[   66.200567]  [<ffffffffa003b4a6>] drm_fb_helper_set_par+0x66/0xe0 [drm_kms_helper]
[   66.200572]  [<ffffffff813e7c03>] fb_set_var+0x283/0x3a0
[   66.200578]  [<ffffffff810a5d05>] ? check_preempt_wakeup+0x165/0x260
[   66.200581]  [<ffffffff810a4751>] ? update_curr+0x141/0x200
[   66.200585]  [<ffffffff8101e3ee>] ? __switch_to_xtra+0x14e/0x180
[   66.200589]  [<ffffffff813f56a4>] fbcon_blank+0x1e4/0x2d0
[   66.200594]  [<ffffffff81475190>] do_unblank_screen.part.17+0xa0/0x180
[   66.200597]  [<ffffffff814752b8>] do_unblank_screen+0x48/0x80
[   66.200601]  [<ffffffff8146a4d5>] complete_change_console+0x65/0xf0
[   66.200605]  [<ffffffff8146b68c>] vt_ioctl+0x112c/0x11d0
[   66.200608]  [<ffffffff810963b3>] ? __wake_up+0x53/0x70
[   66.200613]  [<ffffffff8145ea58>] tty_ioctl+0x298/0x8f0
[   66.200618]  [<ffffffff811dbec9>] ? mntput_no_expire+0x49/0x160
[   66.200621]  [<ffffffff811dc004>] ? mntput+0x24/0x40
[   66.200626]  [<ffffffff811be559>] ? __fput+0x189/0x240
[   66.200630]  [<ffffffff811cf43a>] do_vfs_ioctl+0x7a/0x2e0
[   66.200634]  [<ffffffff811be65e>] ? ____fput+0xe/0x10
[   66.200638]  [<ffffffff810888bc>] ? task_work_run+0xac/0xe0
[   66.200641]  [<ffffffff811cf731>] SyS_ioctl+0x91/0xb0
[   66.200647]  [<ffffffff81014e17>] ? do_notify_resume+0xc7/0xd0
[   66.200651]  [<ffffffff8174f7ad>] system_call_fastpath+0x1a/0x1f
[   66.200680] Code: 4e 08 eb d7 0f 1f 80 00 00 00 00 48 c7 c2 d0 a3 11 a0 48 c7 c6 6e b8 11 a0 31 c0 e8 ab 42 00 00 48 83 c4 18 5b 5d c3 31 d2 eb b0 <0f> 0b 66 66 66 66 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 
[   66.200687] RIP  [<ffffffffa01114e0>] qxl_send_monitors_config+0x150/0x160 [qxl]
[   66.200688]  RSP <ffff880078cd3698>
[   66.200691] ---[ end trace ff52923c1ba99f25 ]---