Clean up scenario functions

Some functions in scenario test cases are redundant. This commit cleans
up them.

Change-Id: I91cd48e8f4c5f152b85e7590eb328814ba8015fa
Add qcow2 image support

This commit adds qcow2 image format for some scenario test. Because
qcow2 disk image format is nicer than uec format.

Change-Id: I4c812176ed1800052c3192110067b1ae7fb04b85
Merge "Clean up scenario functions"
Merge "Add qcow2 image support"