qemu.tests.pci_devices: New test plus qemu_devices updates

Hi guys,

this large patchset is on top of the https://github.com/autotest/virt-test/pull/910 and covers multiple qemu_devices bugfixes, adds support for multifunction devices into QPCIBus and fixes qtree.

All this work was done in order to be able to compare qtree and qdev representations, which should simplify the hotplug testing. Currently the qtree vs. qdev verification is done directly in the pci_devices. Once it's settled I'll move it into the framework and we'll be able to call it after hotplug/unplug and verify that the representation matches.

Please read patch descriptions for details.

Kind regards,
Lukáš