Accept both name and path of the loop device in loop_teardown

The path can be both easily distinguished and constructed from the name.
Add some basic high-level tests

Since we support GObject introspection we can nicely make use of it for testing
and write and run high-level tests in Python. This way we test the
instrospection as well as the library's and plugins' functionality.
First steps to the libbd_utils library

Library containing our utility functions will be useful for many plugins and
possibly even for somebody else in the outer world.
Add missing gobject-2.0 build args to the test-library target