contrib: Add run-kvm-unit-test script

[ldoktor]

This script runs kvm-unit-test tests as individual tests in avocado.

I'll update the avocado-vt documentation to use this script to run kvm-unit-test. When we implement JobAPI we can turn this script to job instead.

[clebergnu]

Usability report:

• Worked first time I tried, good stuff!
• Tests name should, if at all possible, drop the test/ prefix, that is, instead of tests/access, it should be access
• Contrib script should have excutable permissions by default

Now unto code review....