For instance, -device scsi-disk,drive=foo -device scsi-disk,drive=foo
happily creates two SCSI disks connected to the same block device.
It's all downhill from there.

Device usb-storage deliberately attaches twice to the same blockdev,
which fails with the fix in place.  Detach before the second attach
there.

Also catch attempt to delete while a guest device model is attached.
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>