libvirt: remove retval from libvirt _set_host_enabled()

This is an internal method since commit e1fdd8f and the return value
is currently unused, so remove it.

Change-Id: I63f4ea7fdb5218c985cad73e7c795b4329f7b4c9
Merge "libvirt: remove retval from libvirt _set_host_enabled()"