Add write_partition_layout() used to populate the
final image.

Fix duplicated mkimg(1) call in vm_create_disk().

Add primitive (untested) PowerPC/PowerPC64 VM image
support.

Note: As it is currently written, the /boot/pmbr and
/boot/{gptboot,boot1.hfs} use the build host and not
the target build.  Fixing this is likely going to be
a hack in itself.

Sponsored by:	The FreeBSD Foundation