nbd: Allow unmap and fua during write zeroes

nbd: Allow unmap and fua during write zeroes

Commit fa778fff wired up support to send the NBD_CMD_WRITE_ZEROES,
but forgot to inform the block layer that FUA unmapping of zeroes is
supported.  Without BDRV_REQ_MAY_UNMAP listed as a supported flag,
the block layer will always insist on the NBD layer passing
NBD_CMD_FLAG_NO_HOLE, resulting in the server always allocating
things even when it was desired to let the server punch holes.
Similarly, failing to set BDRV_REQ_FUA means that the client may
send unnecessary NBD_CMD_FLUSH when it could have instead used the
NBD_CMD_FLAG_FUA bit.

CC: [email protected]
Signed-off-by: Eric Blake <[email protected]>
Message-Id: <1479413642[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>