tests/lvm: fix checking lvm pool existence cont.

Commit 15cf593bc5 "tests/lvm: fix checking
lvm pool existence" attempted to fix handling '-' in pool name by using
/dev/VG/LV symlink. But those are not created for thin pools. Change
back to /dev/mapper, but include '-' mangling.

Related QubesOS/qubes-issues#4332

qubes/tests/storage_lvm.py

@@ -42,7 +42,9 @@ else:
 
 def lvm_pool_exists(volume_group, thin_pool):
     ''' Returns ``True`` if thin pool exists in the volume group. '''
-    path = "/dev/{!s}/{!s}".format(volume_group, thin_pool)
+    path = "/dev/mapper/{!s}-{!s}".format(
+        volume_group.replace('-', '--'),
+        thin_pool.replace('-', '--'))
     return os.path.exists(path)