tests/lvm: Don't hard-code /dev/vda in test.

Hard-coding it breaks virtio-scsi because the devices are called
/dev/sda etc.
(cherry picked from commit 7509cdf18e)

tests/lvm/test-lvm-mapping.pl

@@ -81,7 +81,7 @@ foreach my $uuid (@lvuuids_in_VG) {
 @lvs_in_VG = sort @lvs_in_VG;
 
 unless (@pvs_in_VG == 2 &&
-        $pvs_in_VG[0] eq "/dev/vda1" && $pvs_in_VG[1] eq "/dev/vda2") {
+        $pvs_in_VG[0] =~ m{/dev/.da1} && $pvs_in_VG[1] =~ m{/dev/.da2}) {
     die "unexpected set of PVs for volume group VG: [",
       join (", ", @pvs_in_VG), "]\n"
 }