Using staticmethod to mock LibvirtDriver._supports_direct_io

The actual test is wrong, but does not failed because the signature
of the fake method "connection_supports_direct_io_stub" accepts an
ulimited number of args.

Change-Id: Ide3e093911723f787d9e84797be86fb7d0cd00c7
Merge "Using staticmethod to mock LibvirtDriver._supports_direct_io"