Allow HostState to handle empty capabilities

In some cases, where the capabilities reported from driver is
emtpy, scheduler would raise exception thus unable to correctly
handle scheduling request even when other backend candidates are
able to fulfill the request.  This change does some modifications
to HostState so that it is able to handle empty capabilities.

Change-Id: Icb6522852d427dea3fbd1ec4b4cc94a184d3669d
Fixes-bug: #1398875
Merge "Allow HostState to handle empty capabilities"