qemu.tests.pci_devices: Style cleanup

Split functions into subfunctions and prepare the test for multiple
parts with exception in the end.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
qemu.tests.pci_devices: Add verification lspci vs. qtree

Additional check for (not only) correct pci addresses in qtree and lspci.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
qemu.tests.pci_devices: Add docstrings/pylint

Add some docstrings and other pylint fixes.

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