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.

[1] https://cloud.google.com/compute/docs/networking/using-gvnic#create_a_vm_with_gvnic_support

Reviewed by: cperciva
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D43411