Fix launching libvirt instances with swap

The code in

  commit 7be531fe9462f2b07d4a1abf6687f649d1dfbb89
  Author: Daniel P. Berrange <berrange@redhat.com>
  Date:   Tue Jan 15 20:43:48 2013 +0000

    Refactor code for setting up libvirt disk mappings

accidentally broken launching of instances where
the flavor had non-zero swap specified. This was
not caught due to complete lack of test coverage
for the '_create_image' method. This adds two
test cases for that method, exercising this bug.
It also fixes a bug in netutils which was not
handling a NULL network mapping, which is a valid
scenario for several callers.

Fixes bug #1131306
Change-Id: I214561ba3ec6440d11a5afb0665e6b6f990bd148
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Merge "Fix launching libvirt instances with swap"