fix server_multi postinstall for disk being virtio

Summary:
With the previous change to the server_(sata)_multi test, we
need to adjust the post-install test to use vdb not sdb, the
disks are virtIO now (not PATA as they were with 4.2)

Test Plan:
Check the server_multi test actually completes
properly now

Reviewers: garretraziel, jskladan

Reviewed By: jskladan

Subscribers: tflink

Differential Revision: https://phab.qadevel.cloud.fedoraproject.org/D732

tests/disk_guided_multi_postinstall.pm

@@ -5,7 +5,7 @@ use testapi;
 sub run {
     assert_screen "root_console";
     # check that second disk is intact
-    assert_script_run 'mount /dev/sdb1 /mnt';
+    assert_script_run 'mount /dev/vdb1 /mnt';
     validate_script_output 'cat /mnt/testfile', sub { $_ =~ m/Hello, world!/ };
 }