Adds retry on upload_vhd for xapi glance plugin

Add a retry for the xapi glance plugin to handle transient issues
when uploading the vhd.

An example issue we're seeing is a connection timeout:

['XENAPI_PLUGIN_FAILURE', 'upload_vhd', 'error',
		"(110, 'Connection timed out')"]

To work around transient issues such as a connection timeout, we
should retry based on glance_num_retries before outright failing.

Change-Id: Ice6fdd3dd39ef40e5997d69209aaafa66bff5d6e
Fixes: bug #1134493
Merge "Adds retry on upload_vhd for xapi glance plugin"