apparmor: allow to call vhost-user-gpu

Configuring vhost-user-gpu like:
<video>
<driver name='vhostuser'/>
<model type='virtio' heads='1'/>
</video>
Triggers an apparmor denial like:
apparmor="DENIED" operation="exec" profile="libvirtd"
name="/usr/lib/qemu/vhost-user-gpu" pid=888257 comm="libvirtd"
requested_mask="x" denied_mask="x" fsuid=0 ouid=0

This helper is provided by qemu for vhost-user-gpu and thereby being
in the same path as qemu_bridge_helper. Due to that adding a rule allowing
to call uses the same path list.

Reviewed-by: Jim Fehlig <jfehlig@suse.com>
Signed-off-by: Christian Ehrhardt <christian.ehrhardt@canonical.com>