qemu: ensure vhostuser FD is initialized to -1

qemu: ensure vhostuser FD is initialized to -1

The video private data was not initializing the vhostuser FD
causing us to attempt to close FD 0 many times over.

Fixes

  commit ca60ecfa8cc1bd85baf7137dd1864d5f00f019f0
  Author: Marc-André Lureau <marcandre.lureau@redhat.com>
  Date:   Mon Sep 23 14:44:36 2019 +0400

      qemu: add qemuDomainVideoPrivate

Since the test suite does not invoke qemuExtDevicesStart(), no
vhost_user_fd will be present when generating test XML. To deal
with this we can must a fake FD number. While the current XML
is using FD == 0, we pick a very interesting number that's unlikely
to be a real FD, so that we're more likely to see any mistakes
closing the invalid FD.

Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>