releng-gce: Advertise the availability of gVNIC support in GCE images.
This marks FreeBSD GCE images as gVNIC capable by adding the
--guest-os-features=GVNIC flag at creation time as suggested in GCE
documentation[1]. This allows Generation 3 and newer GCE instances
to leverage advanced networking capabilities and performance
enhancements provided by gVNIC. Users will benefit from these
improvements without needing to create custom images.