qemu_snapshot: error out when deleting internal snapshot on non-active disk

qemu_snapshot: error out when deleting internal snapshot on non-active disk

Deleting internal snapshot when the currently active disk image is
different than where the internal snapshot was taken doesn't work
correctly.

This applies to a running VM only as we are using QMP command and
talking to the QEMU process that is using different disk.

This works correctly when the VM is shut of as in this case we spawn
qemu-img process to delete the snapshot.

Signed-off-by: Pavel Hrdina <phrdina@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>