vhost-user-bridge: fix resume regression (since 2.9)

vhost-user-bridge: fix resume regression (since 2.9)

Commit e10e798c85c2331 switched to libvhost-user which lacked support
for resuming the avail_idx based on used_idx.

Fixes:
https://bugzilla.redhat.com/show_bug.cgi?id=1485867

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
(cherry picked from commit 672339f7eff5e9226f302037290e84e783d2b5cd)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>