Add a new BDRV_REQ_MASK constant, and use it to make sure that
caller flags are always valid.

Tested with 'make check' and with qemu-iotests on both '-raw'
and '-qcow2'; the only failure turned up was fixed in the
previous commit.
Signed-off-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>