Add support for cross-building cloudware images.

If MACHINE_ARCH doesn't match TARGET_ARCH, and we're not in the special
case of building i386 images on an amd64 host, we need to pull in the
qemu-user-static package; this allows us to run some commands inside
the VM disk image chroot, most notably to install packages.

Reviewed by: gjb
MFC after: 2 weeks
Sponsored by: FreeBSD/EC2 patreon (https://www.patreon.com/cperciva)