Hi James,
  I agree it's likely to be an lvm/udev race; some observations:
     1) It doesn't seem to happen with files
     2) With lvm, It doesn't always happen if the guest is configured with a single core; most of the time but not always
     3) With lvm it seems to happen (almost?) always when the guest has multiple cores.

output of tgtadm as requested:

----- lvm based, bad ---

Target 1: iqn.2008-09.com.example:server1.share1
    System information:
        Driver: iscsi
        State: ready
    I_T nexus information:
        I_T nexus: 1
            Initiator: iqn.2005-03.org.open-iscsi:2b2883cd283
            Connection: 0
                IP Address: 192.168.123.12
        I_T nexus: 2
            Initiator: iqn.2005-03.org.open-iscsi:2b2883cd282
            Connection: 0
                IP Address: 192.168.122.12
    LUN information:
        LUN: 0
            Type: controller
            SCSI ID: IET     00010000
            SCSI SN: beaf10
            Size: 0 MB, Block size: 1
            Online: Yes
            Removable media: No
            Readonly: No
            Backing store type: null
            Backing store path: None
            Backing store flags: 
    Account information:
    ACL information:
        ALL

--- LVM based, after restarting tgt ---

Target 1: iqn.2008-09.com.example:server1.share1
    System information:
        Driver: iscsi
        State: ready
    I_T nexus information:
        I_T nexus: 1
            Initiator: iqn.2005-03.org.open-iscsi:2b2883cd283
            Connection: 0
                IP Address: 192.168.123.12
        I_T nexus: 2
            Initiator: iqn.2005-03.org.open-iscsi:2b2883cd282
            Connection: 0
                IP Address: 192.168.122.12
        I_T nexus: 3
            Initiator: iqn.2005-03.org.open-iscsi:2b2883cd283
            Connection: 0
                IP Address: 192.168.123.12
        I_T nexus: 4
            Initiator: iqn.2005-03.org.open-iscsi:2b2883cd282
            Connection: 0
                IP Address: 192.168.122.12
    LUN information:
        LUN: 0
            Type: controller
            SCSI ID: IET     00010000
            SCSI SN: beaf10
            Size: 0 MB, Block size: 1
            Online: Yes
            Removable media: No
            Readonly: No
            Backing store type: null
            Backing store path: None
            Backing store flags: 
        LUN: 1
            Type: disk
            SCSI ID: IET     00010001
            SCSI SN: beaf11
            Size: 10737 MB, Block size: 512
            Online: Yes
            Removable media: No
            Readonly: No
            Backing store type: rdwr
            Backing store path: /dev/mapper/server1-share1
            Backing store flags: 
    Account information:
    ACL information:
        ALL

(can also see the client accessing it there)

--- file based, good ---

Target 1: iqn.2008-09.com.example:server1.share1
    System information:
        Driver: iscsi
        State: ready
    I_T nexus information:
        I_T nexus: 5
            Initiator: iqn.2005-03.org.open-iscsi:2b2883cd283
            Connection: 0
                IP Address: 192.168.123.12
        I_T nexus: 6
            Initiator: iqn.2005-03.org.open-iscsi:2b2883cd282
            Connection: 0
                IP Address: 192.168.122.12
    LUN information:
        LUN: 0
            Type: controller
            SCSI ID: IET     00010000
            SCSI SN: beaf10
            Size: 0 MB, Block size: 1
            Online: Yes
            Removable media: No
            Readonly: No
            Backing store type: null
            Backing store path: None
            Backing store flags: 
        LUN: 1
            Type: disk
            SCSI ID: IET     00010001
            SCSI SN: beaf11
            Size: 1074 MB, Block size: 512
            Online: Yes
            Removable media: No
            Readonly: No
            Backing store type: rdwr
            Backing store path: /home/dg/afile
            Backing store flags: 
    Account information:
    ACL information:
        ALL