Raise an exception if qemu-img fails

Raise an exception if the qemu image doesn't
exist or qemu-img fails.

Co-Authored-By: Chuck Short <chuck.short@canonical.com>
Closes-Bug: #1168318
Change-Id: I6b4123590e7d2934de0bc6add900d708d5986039
Merge "Raise an exception if qemu-img fails"