virttest.qemu_devices: Add qemu_devices and use it for qemu VM

This patch modifies the way how qemu_command is created. Instead
of long string it creates a representation of devices, which behaves
similarily to qemu's qdev.

Current implementation is very limited and only handles PCI bus. All
devices are created as simple StringDevices so the benefit might
not be obvious. But later patches adds specific devices (usb, image,
...) and their specific behaviour.

Additionally this patch adds the basic support for q35 machine.

Later the support for automatic check of running VM and representation
will be added.

NOTE: This version's pci addresses are not assigned properly as
      upstream currently uses different method. Following patch
      removes the original mechanism and uses the one from this patch.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>