<not/> element in XML filters

This turns out to be an elegant solution (thanks ncoghlan!) for
expressing negative filters on one-to-many relationships, which are
currently handled by special-casing the != operator in some elements
such as <group/>.

Change-Id: I56596f03ef41e7d82a768f080f03b3c33cbf9305
allow searching by disk details in web UI and hostRequires

Bug: 766919
Change-Id: I8117efa0711882a5fd097374bd8eff7401a6a260
support units="" attribute on <disk/> size filters

Bug: 766919
Change-Id: Ic347de86b5d7c443f231f535e984fc1a15145763
Merge changes Ic347de86,I8117efa0,I56596f03 into develop

* changes:
  support units="" attribute on <disk/> size filters
  allow searching by disk details in web UI and hostRequires
  <not/> element in XML filters