Use "virtio_blk" in "virtio_scsi_mq" test for boot dev

The "virtio_blk" is generally usable nowadays while it should not clash
with the "virtio_scsi" used in the test. Let's use it to boot the
machine instead of ide/ahci, which is not supported on all archs.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>

qemu/tests/cfg/virtio_scsi_mq.cfg

@@ -9,5 +9,5 @@
     start_vm = no
     image_size_extra_images = 512M
     q35:
-        system_image_drive_format = ahci
         dev_type = q35-pcihost
+    system_image_drive_format = virtio_blk

qemu/tests/virtio_scsi_mq.py

@@ -65,7 +65,8 @@ def run(test, params, env):
     images_num = int(num_queues)
     extra_image_size = params.get("image_size_extra_images", "512M")
     system_image = params.get("images")
-    system_image_drive_format = params.get("system_image_drive_format", "ide")
+    system_image_drive_format = params.get("system_image_drive_format",
+                                           "virtio_blk")
     params["drive_format_%s" % system_image] = system_image_drive_format
 
     error.context("Boot up guest with block devcie with num_queues"