vhost: enable reply-ack systematically

vhost: enable reply-ack systematically

As announced during v20.05 release cycle, this
patch makes reply-ack protocol feature to be enabled
unconditionally.

This protocol feature makes the communication between the
master and the slave more robust, avoiding for example
possible undefined behaviour with VHOST_USER_SET_MEM_TABLE.

Also, reply-ack support will be required for upcoming
VHOST_USER_SET_STATUS request.

Note that this protocol feature was disabled by default
because Qemu version 2.7.0 to 2.9.0 had a bug causing a
deadlock when reply-ack was negotiated and multiqueue
enabled. These Qemu version are now very old and no more
maintained, so we can reasonably consider we no more
support them.

Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Reviewed-by: Chenbo Xia <chenbo.xia@intel.com>