Fix Cirrus-CI boot smoke test

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

(cherry picked from commit a7d593dd1da27833b5384349700bc3c7bcae6aad)
(cherry picked from commit 982cd962061552fbeb43e3bce2a5d7fd05231dde)
(cherry picked from commit 407abff2b91847e23711625ad7c69c17e99b3d1d)
(cherry picked from commit 87c0db9d0174fb1906a58d2f3bd5aff6dad528de)
(cherry picked from commit cec2682cd2910080951782391d7d7cd30c95c4ea)
(cherry picked from commit 088dbb4b8d3eb8275c9aba34eb521dbc79f98f51)

Note that this does not have any effect on built artifacts (and thus is
not an EN candidate) but allows Cirrus-CI to be used on the releng
branch.

Approved by: so