Removed the duplicated _host_state = None in libvirt driver

Happened to see there was a duplicated value assignment of host state
to None in __init__() in libvirt driver. Removed one of them.

Change-Id: I4d939f301c86b369b8f740bb18c9f3f6c77a674b
Signed-off-by: Shane Wang <shane.wang@intel.com>
Merge "Removed the duplicated _host_state = None in libvirt driver"