Add negotiation flag check for FLUSH

Some older version of qemu-nbd cuases error exit with nbd_client.py.
(Look at: https://groups.google.com/d/msg/osv-dev/EW5BtNFNfzs/I33BeFXg2f0J)
This is because nbd_client.py is sending FLUSH command unconditionally, but it's extended feature, nbd client should check nbd server has the capability to accept FLUSH.
nbd server sends capability flags on negotiation stage, it sends HAS_FLAGS(0x1) and SEND_FLUSH(0x4) when server supports FLUSH.

This patch adds these capability check, and skips to send FLUSH if server doesn't support it.

Signed-off-by: Takuya ASADA <syuu@dokukino.com>
Reviewed-by: Benoît Canet <benoit.canet@irqsave.net>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>