generic.tests: Let lvm get disk names from command

Update the script to get disk names from command instead of the
cfg files. So we don't need to update it when the prefix of
disks changed.
Signed-off-by: NYiqiao Pu <ypu@redhat.com>

generic/tests/cfg/lvm.cfg

@@ -11,7 +11,6 @@
     image_size_stg2 = 1G
     image_format_stg2 = qcow2
     guest_testdir = /mnt
-    disks = "/dev/sdb /dev/sdc"
     kill_vm = no
     post_command_noncritical = no
     variants:

generic/tests/lvm.py

@@ -40,9 +40,11 @@ def run(test, params, env):
     vg_name = "vg_kvm_test"
     lv_name = "lv_kvm_test"
     lv_path = "/dev/%s/%s" % (vg_name, lv_name)
-    disks = params.get("disks", "/dev/hdb /dev/hdc")
     clean = params.get("clean", "yes")
     timeout = params.get("lvm_timeout", "600")
+    output = session.cmd_output("ls /dev/[hvs]da")
+    disk_prefix = output.strip()[:len('/dev/hd')]
+    disks = "%sb %sc" % (disk_prefix, disk_prefix)
 
     try:
         error.context("adding physical volumes %s" % disks, logging.info)