snapshot: create helper to test that block drivers supports snapshots
The patch enforces proper locking for this operation.
Signed-off-by: Denis V. Lunev <den@openvz.org> Reviewed-by: Greg Kurz <gkurz@linux.vnet.ibm.com> Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com> Reviewed-by: Fam Zheng <famz@redhat.com> Reviewed-by: Juan Quintela <quintela@redhat.com> CC: Kevin Wolf <kwolf@redhat.com> Tested-by: Greg Kurz <gkurz@linux.vnet.ibm.com> Signed-off-by: Juan Quintela <quintela@redhat.com>