xenapi: stop injecting the hostname during resize

The windows PV tools will set the hostname of a server to the value
given in the hostname xenstore key. If we inject that during resize, the
users will be unable to change their server hostname, and in addition it
will be reset to the original name.

Fixes bug 1207829
Change-Id: I49ba9d9f4f8b4a86488e427d99f69236bd959c78
Merge "xenapi: stop injecting the hostname during resize"