virttest.qemu_vm: Use get_free_pci_addr() for every PCI device

Add get_free_pci_addr() everywhere, where PCI device is defined. There
are some changes, which were required because qemu doesn't handle all
devices the same way. Default soundcard and autocreated scsi driver
are using lowest possible addresses so it's necessarily to check for
their presense before we start using this function.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
virttest.qemu_vm: Fix cdrom handling

The cdrom handling part of create_cmd was corrupted during rebases.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>