qemu.tests.multi_disk: Fix bootindex bug

Fix signal_repeat variant to boot with the correct bootindex.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
virttest.qemu_qtree: Hook for ahci disks

AHCI disk is in reality ide-drive disk inside AHCI bus.

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