qemu: Allow multiple bridges when pci-bridges is not available

qemu: Allow multiple bridges when pci-bridges is not available

qemuDomainAssignPCIAddresses() hardcoded the assumption
that the only way to support devices on a non-zero bus is
to add one or more pci-bridges; however, since we now
support a large selection of PCI controllers that can be
used instead, the assumption is no longer true.

Moreover, this check was always redundant, because the
only sensible time to check for the availability of
pci-bridge is when building the QEMU command line, and
such a check is of course already in place.

In fact, there were *two* such checks, but since one of
the two was relying on the incorrect assumption explained
above, and it was redundant anyway, it has been dropped.