libvirt: make guestfs methods always return list of tuples

guestfs.GuestFS supports parameter python_return_dict with default
False in 1.22 (backported in 1.20). This indicates that your program
wants to receive Python dicts for methods in the API that return list
of tuples. See http://libguestfs.org/guestfs-release-notes.1.html
In a future version of libguestfs, its default value will be True,
then that will break the code, to avoid this, we'd better force
python_return_dict as False. See http://libguestfs.org/guestfs-python.3.html

This commit makes method inspect_get_mountpoints always return list
of tuples.

Change-Id: I8211cb31a7a890f86cdd818767b3d5e8cfd5bbed
Merge "libvirt: make guestfs methods always return list of tuples"