As with the local SCSI passthrough devicesm qemu can't support snapshots
on those as the block ops are handled by the device. This is also true
for iSCSI backing of the disk. Remove the check for the local block
device and just forbid snapshot when the disk is of type 'lun'.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1073368