check ephemeral disk format at libvirt before use

Add ephemeral disk format at libvirt. There is no check before
so it will be used directly. This patch adds valid check function
and validate it before eph disk was used in virt layer.

Change-Id: I1934573e79f126e725cef4848b15c11a595161c9
Closes-Bug: #1293880
Merge "check ephemeral disk format at libvirt before use"