janitorial: guest monitor: simplify getGuestName()

The get() method of the dictionaries has the same semantic
of the old method, but just one line of code.

Change-Id: I73e2fd6e6eb97cc51d28f70c8ade818fdc74e0be
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/31588
Reviewed-by: Adam Litke <alitke@redhat.com>