qemu: Add check for PCI bridge placement if there are too many PCI devices

qemu: Add check for PCI bridge placement if there are too many PCI devices

Previous patch of this series fixed the issue with adding a new PCI bridge
when all the slots were reserved by devices with user specified addresses.
In case there are still some PCI devices waiting to get a slot reserved
by qemuAssignDevicePCISlots, this means a new bus needs to be
created along with a corresponding bridge controller. By adding an
additional check, this scenario now results in a reasonable error
instead of generating wrong qemu command line.