xenapi: more info when assert_can_migrate fails

VM.assert_can_migrate can fail for many reasons. This makes it hard to
debug, so lets ensure we report the reason we get back from XenServer in
the exception that gets raised and saved into the instance_fault.

Change-Id: I3deb1169d6fe797ca74cd89061bbcf53d416e8d6
Merge "xenapi: more info when assert_can_migrate fails"