qemu.tests.multi_disk: Use images/$disk_name location by default

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
qemu.tests.multi_disk: Fix bug in max_disk.ide configuration

Current config generates 5 disks, because one is added by the test
logic. This can be workarounded by setting stg_image_num = 0 and
using old params or using stg_params instead (this patch).

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
qemu.tests.multi_disk: Don't print monitor help to log

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
qemu.tests.multi_disk: start from PCI 3 in max_disk.virtio_blk

For Linux guests we require 27 disk + 1 boot image. We have to start
assigning PCIs from 0x3 in order to have 1 slot for NIC.

It would be also possible to remove VGA and use 28 disks.

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