release/Makefile.vm: Support read-only ports tree

Build qemu (if needed) with WRKDIRPREFIX=/tmp/ports DISTDIR=/tmp/distfiles
so that we can have a read-only /usr/ports and don't contaminate it. This
became an issue when I enabled parallel release building, since one image
might be creating its ports.txz file at the same time as we're building
qemu as a prerequisite for building another image.

MFC after: 5 days

(cherry picked from commit 6f454c211b295ad13e80fc5b858efc9bad33e33b)