libvirt: add _get_launch_flags helper method in unit test

In libvirt driver , we have some logic to generate launch_flags in
method _create_domain_and_network, but in unit test we hard code
launch_flags with '0'. This may lead unit test failure in actual test.
We can't extract the logic to a method in libvirt/driver.py, so add
a helper method with same logic for unit test.

Change-Id: I39ce0162b856daf18488e006b0e424c11811e0c0
Merge "libvirt: add _get_launch_flags helper method in unit test"