If we can't probe the architecture from QMP we parse the architecture
from the qemu binaries name. This results in the architecture being i386
instead of i686 which then results in QEMU_CAPS_PCI_MULTIBUS being unset
which gives a broken qemu command line.

This probably didn't show up earlier since most of the time there's also
a /usr/bin/qemu around which results in i686 capabilities.