Update hypervisor_hostname after live migration

After a live migration, the OS-EXT-SRV-ATTR:hypervisor_hostname
attribute of the instance is not updated correctly.  This change
adds a parameter to the call to _instance_update that fixes the
problem.

Fixes bug 1173376

Change-Id: I40f1ead2d93c7a9979a914e0fb96e3e5bb287b48
Merge "Update hypervisor_hostname after live migration"