qemu: snapshot: Forbid snapshots of iSCSI passthrough devices

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

src/qemu/qemu_driver.c

@@ -12434,10 +12434,7 @@ qemuDomainSnapshotPrepareDiskExternalBackingInactive(virDomainDiskDefPtr disk)
 static int
 qemuDomainSnapshotPrepareDiskExternalBackingActive(virDomainDiskDefPtr disk)
 {
-    int actualType = virStorageSourceGetActualType(disk->src);
-
-    if (actualType == VIR_STORAGE_TYPE_BLOCK &&
-        disk->device == VIR_DOMAIN_DISK_DEVICE_LUN) {
+    if (disk->device == VIR_DOMAIN_DISK_DEVICE_LUN) {
         virReportError(VIR_ERR_CONFIG_UNSUPPORTED, "%s",
                        _("external active snapshots are not supported on scsi "
                          "passthrough devices"));