Restore Cirrus-CI boot smoke test

This reverts commit a7d593dd1da27833b5384349700bc3c7bcae6aad.
We now use compute_engine_instance which allows us to specify a custom
disk size. Also go back to using the default qemu version (rather than
qemu42 or qemu-devel) as any issues were fixed some time ago.

Reviewed by: lwhsu, markj
MFC after: 1 month
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D30082