libvirt: remove pointless HostState class

Now that the get_host_stats method is removed, the only user
of the HostState class passes 'refresh=True' every time. Given
this, there is no point in caching the data in the HostState
class. Instead just populate the data directly in the
get_available_resources method.

Blueprint: virt-driver-get-available-resources-object
Change-Id: I19c391a1bae27c3e8d891ecf77ceeaf170299be3
Merge "libvirt: remove pointless HostState class"