xenapi: check for IP address in live migration pre check

A requirement for live migration is that the destination
host contains a management PIF with an IP address assigned.
If a host is broken (no IP), the compute log simply shows a generic
exception on the xenapi call host.migrate_receive.  We can
provide better information that the IP address is missing
by checking this prior to the call.

Closes-Bug: 1245592

Change-Id: Iaa6345f4bddf2f98efb4a5c9743da16ba8ae7b56
Merge "xenapi: check for IP address in live migration pre check"