qemu: don't assume slot 0 is unused/reserved.

qemu: don't assume slot 0 is unused/reserved.

When qemuAssignDevicePCISlots() is looking for companion controllers
for a USB controller that has no PCI address specified, it initializes
a virDevicePCIAddress to 0000:00:00.0, fills it in with the
companion's address if one is found, then checks whether or not there
was a find based on slot == 0. On a system with a single PCI bus, that
is a valid way to check, because slot 0 is reserved, but on most other
PCI buses, slot 0 is not reserved, and is open for use by any
device. This patch adds a separate bool that is set when a companion
is found rather than relying on the faulty information provided with
"slot == 0".