qemu snapshot: use QMP snapshot-save for internal snapshots creation

qemu snapshot: use QMP snapshot-save for internal snapshots creation

The usage of HMP commands are highly discouraged by qemu. Moreover,
current snapshot creation routine does not provide flexibility in
choosing target device for VM state snapshot.

This patch makes use of QMP commands snapshot-save and by
default chooses first writable non-shared qcow2 disk (if present)
as target for VM state.

Signed-off-by: Nikolai Barybin <nikolai.barybin@virtuozzo.com>
Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Pavel Hrdina <phrdina@redhat.com>