Format eph disk with  specified format in libvirt

novaclient has following command parameters:
--ephemeral size=<size>[,format=<format>]
Create and attach a local ephemeral block device of
<size> GB and format it to <format>.

so mkfs should use the specified format instead of using
default format.

Change-Id: I7a8753284d7b1da1a1203e85b430bd0c5012937a
Closes-Bug: #1280132
Merge "Format eph disk with  specified format in libvirt"