support partitions on loop devices

loop device parsing should have "dXpY_style = True" in order to
correctly parse partitions on loop devices.

Reasoning:
==========
Using losetup to create a virtual SD card disk into a loop device and
creating partitions for it results in new devices within an AppVM that
look like: /dev/loop0p1 /dev/loop0p2 and so on.

However as soon as they are created, Qubes Manager rises an exception
and becomes blocked with the following message (redacted):
"QubesException: Invalid device name: loop0p1
at line 639 of file /usr/lib64/python2.7/site-
packages/qubesmanager/main.py

Details:
line: raise QubesException....
func: block_name_to_majorminor
line no.: 181
file: ....../qubes/qubesutils.py

core/qubesutils.py

@@ -161,6 +161,7 @@ def block_name_to_majorminor(name):
         disk = False
         major = 11
     elif name.startswith("loop"):
+        dXpY_style = True
         disk = False
         major = 7
     elif name.startswith("md"):