release/vm: add a WITHOUT_QEMU option

This option disables trying to build a qemu-<target>-static and simply
doesn't do any of the operations that rely on it. This disables package
installation which is required by most, but not all image types.

Disable all CLOUDWARE variants that require pkg when WITHOUT_QEMU is set
to avoid broken images.

Reviewed by: emaste
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D49310