qemu_cleanups: set crda REGDOMAIN as intended based on distro11s.conf

Previously, running qemu-cleanups.sh would append the
DISTRO11S_REGDOMAIN instead of replacing it.  So, if you
ran qemu_cleanups.sh three times you'd have:
"REG_DOMAIN=USUSUS" in /etc/default/crda

This fixes the regex so that you'll end up with:
"REG_DOMAIN=US"