qemu: don't always reserve PCI addresses for implicit controllers

qemu: don't always reserve PCI addresses for implicit controllers

In the past we automatically added a USB controller and assigned
it a PCI address (0:0:1.2) even on machines without a PCI bus.
This didn't break machines with no PCI bus  because the command
line for it is just '-usb', with no mention of the PCI bus.

The implicit IDE controller (reserved address 0:0:1.1) has
no command line at all.

Commit b33eb0dc removed the ability to reserve PCI addresses
on machines without a PCI bus. This made them stop working,
since there would always be the implicit USB controller.

Skip the reservation of addresses for these controllers when
there is no PCI bus, instead of failing.