qemu.tests.pci_devices: Fix the add_devices_all problem and login to VM

add_devices_all occupied all ports even when the bus was used for another
buses. This caused problems with occupied ports. This patch increases
the first_port number of the upper bus with each inserted bus thus the
bus could be plugged there.

The drawback is that buses which has some buses inside will have those
buses on the first addresses and the testing devices would not start with
addr=0x0. On the other hand missing bus would also test that the bus is
incapable of handling the first slot correctly.

Additionally this patch adds wait_for_login() in order to verify that
the VM booted correctly.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>