Similar to the qemu_tpm.c, add a unit with a few functions to
start/stop and setup the cgroup of the external vhost-user-gpu
process. See function documentation.

The vhost-user connection fd is set on qemuDomainVideoPrivate struct.
Signed-off-by: NMarc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: NCole Robinson <crobinso@redhat.com>