virsh snapshot-create-as supports 'file' storage type in --diskspec by default.
But it doesn't support 'block' storage type in the virshParseSnapshotDiskspec().
So if a snapshot on a block device (e.g. LV) was created, the type of
current running storage source in dumpxml is inconsistent with the actual
backend storage source. It will check file-system type mismatch failed
and return an error message of 'Migration without shared storage is unsafe'
when VM performs a live migration after this snapshot.

Considering virsh has to be able to work remotely that recognizing a block device
by prefix /dev/ or by stat() may be not suitable, so adding a "stype" field
for the --diskspec string which will be either "file" or "block".
e.g. --diskspec vda,snapshot=external,driver=qcow2,stype=block,file=/dev/xxx.
Signed-off-by: NLiu Dayu <liu.dayu@zte.com.cn>
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>