install_prereq: Fix check_installed_debs remove subversion

check_installed_debs wasn't handling virtual packages like libsrtp-dev and
libresample-dev and on multiarch systems it was accidentally filtering out all
packages if any :i386 packages were found instead of just filtering out the
:i386 packages themselves.

Change-Id: Ifd68da0d1ee30cc84df14de3f9b9079d7c3cecda