vhost-user-blk: prevent using uninitialized vqs

vhost-user-blk: prevent using uninitialized vqs

Same rational as: e6cc11d64fc998c11a4dfcde8fda3fc33a74d844

Of the 3 virtqueues, seabios only sets cmd, leaving ctrl
and event without a physical address. This can cause
vhost_verify_ring_part_mapping to return ENOMEM, causing
the following logs:

qemu-system-x86_64: Unable to map available ring for ring 0
qemu-system-x86_64: Verify ring failure on region 0

This has already been fixed for vhost scsi devices and was
recently vhost-user scsi devices. This commit fixes it for
vhost-user-blk devices.

Suggested-by: Phillippe Mathieu-Daude <philmd@redhat.com>
Signed-off-by: Raphael Norwitz <raphael.norwitz@nutanix.com>
Message-Id: <1566498865-55506-1-git-send-email-raphael.norwitz@nutanix.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>