virttest.qemu_vm: Handle PCIs by qemu_devices

This patch adds two types of qemu_devices and uses them for PCI
devices. The addr and bus items are handled by qemu_devices bus.

New config option is also introduced:
    strict_mode(bool): which enforce setting optional parameters
                       (bus, addr, ...)
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>