Add commands to query Virtual Machine Generation ID counter.

QMP command example:
    { "execute": "query-vm-generation-id" }

HMP command example:
    info vm-generation-id
Signed-off-by: NIgor Mammedov <imammedo@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NBen Warren <ben@skyportsystems.com>
Reviewed-by: NLaszlo Ersek <lersek@redhat.com>
Tested-by: NLaszlo Ersek <lersek@redhat.com>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>