virttest.qemu_devices: Split into separate submodules

Hi guys,

this patchset is on top of the https://github.com/autotest/virt-test/pull/1222 pull request. Please focus primarily on the commits `virttest.qemu_devices: Split qemu_devices into separate submodules`. The remaining two patches are minor unrelated fixups found while testing these changes.

Kind regards,
Lukáš