Fix #1717: delete snapshot when vm running and drive not attached

Fix #1717: delete snapshot when vm running and drive not attached

changelog v2:
 - remove hash
 - remove check if cdrom

if we try to delete a snapshot, and that is disk from the snapshot
is not attached anymore (unused), we can't delete the snapshot
with qemu snapshot delete command (for storage which use it (qcow2,rbd,...))

example:

...
unused0: rbd:vm-107-disk-3

[snap1]
...
scsi2: rbd:vm-107-disk-3,size=1G

-> die
 qmp command 'delete-drive-snapshot' failed - Device 'drive-scsi2' not found

If drive is not attached, we need to use the storage snapshot delete command