Add missing tests for db.service_* methods

There was no tests in test_db_api for service_* methods.

Add tests to ensure that all works. Also these tests allow us to refactor
work with session in these methods.

blueprint db-api-tests

Change-Id: I0c3b233e1290f08ea01a8c64eacb95af1eae6f24
Fix missing argument to QemuImageInfo

set the default value for cmd_output to None. _parse
does work fine with None or empty string

Fixes LP# 1157412

Change-Id: Ib63f2fd227633b96db613ce29563d1929536819a
Merge "Add missing tests for db.service_* methods"
Merge "Fix missing argument to QemuImageInfo"