This patch reworks blkdev_name_to_number to allow the tools to attach xvd disks
beyond xvdp.  Once this patch is applied, the whole system should work together
and allow disks xvdq onwards.  The patch does 2 things:

1.  Adds the handling for the /dev/xvd[q-z] and /dev/xvd[a-i][a-z] extended devices

2.  Changes blkdev_name_to_number() to return a tuple of (xenbus, devnum), and
then deals with the resulting fallout.

Changes since v1:
- Use a more pythonic way in main.py to get the vbd_devids

Signed-off-by: Chris Lalancette <clalance@redhat.com>