Add hypervisor information extension.

Adds a new extension to get information about hypervisors (as opposed
to compute hosts), including a list of hypervisors matching a regular
expression (database regular expression, i.e., %'s) and a list of
hypervisors with the list of instances living on those hypervisors.

Change-Id: I2e44e82e123e5e727b49622fe5e966131658f9f6