iotests/262: Switch source/dest VM launch order

Launching the destination VM before the source VM gives us a regression
test for HEAD^:

The guest device causes a read from the disk image through
guess_disk_lchs().  This will not work if the first sector (containing
the partition table) is yet unallocated, we use COR, and the node is
inactive.

By launching the source VM before the destination, however, the COR
filter on the source will allocate that area in the image shared between
both VMs, thus the problem will not become apparent.

Switching the launch order causes the sector to still be unallocated
when guess_disk_lchs() runs on the inactive node in the destination VM,
and thus we get our test case.
Signed-off-by: NMax Reitz <mreitz@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Message-id: 20191001174827.11081-3-mreitz@redhat.com
Message-Id: <20191001174827.11081-3-mreitz@redhat.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

tests/qemu-iotests/262

@@ -54,12 +54,6 @@ with iotests.FilePath('img') as img_path, \
 
     os.mkfifo(fifo)
 
-    iotests.log('Launching source VM...')
-    add_opts(vm_a)
-    vm_a.launch()
-
-    vm_a.enable_migration_events('A')
-
     iotests.log('Launching destination VM...')
     add_opts(vm_b)
     vm_b.add_incoming("exec: cat '%s'" % (fifo))
@@ -67,6 +61,12 @@ with iotests.FilePath('img') as img_path, \
 
     vm_b.enable_migration_events('B')
 
+    iotests.log('Launching source VM...')
+    add_opts(vm_a)
+    vm_a.launch()
+
+    vm_a.enable_migration_events('A')
+
     iotests.log('Starting migration to B...')
     iotests.log(vm_a.qmp('migrate', uri='exec:cat >%s' % (fifo)))
     with iotests.Timeout(3, 'Migration does not complete'):

tests/qemu-iotests/262.out

-Launching source VM...
-Enabling migration QMP events on A...
-{"return": {}}
 Launching destination VM...
 Enabling migration QMP events on B...
 {"return": {}}
+Launching source VM...
+Enabling migration QMP events on A...
+{"return": {}}
 Starting migration to B...
 {"return": {}}
 {"data": {"status": "setup"}, "event": "MIGRATION", "timestamp": {"microseconds": "USECS", "seconds": "SECS"}}