vhost-user-common: send get_inflight_fd once

vhost-user-common: send get_inflight_fd once

Currently the get_inflight_fd will be sent every time the device is started, and
the backend will allocate shared memory to save the inflight state. If the
backend finds that it receives the second get_inflight_fd, it will release the
previous shared memory, which breaks inflight working logic.

This patch is a preparation for the following patches.

Signed-off-by: Li Feng <fengli@smartx.com>
Reviewed-by: Raphael Norwitz <raphael.norwitz@nutanix.com>
Message-Id: <20231009044735.941655-2-fengli@smartx.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>