CA-140647: Customer is not getting the list of updates when ever they se...

...lect the "Check for Updates" option from the tools menu within Xencenter.

-Added 'Host:' to HTTP headers for all the HTTP GET and PUT requests sent by XenCenter.
This change will presumably fix network issues occurring when going through certain corporate firewalls/proxies.

Signed-off-by: Gabor Apati-Nagy <gabor.apati-nagy@citrix.com>