xenapi: Improve error message on session failure

xenapi: Improve error message on session failure

XenAPI session login can fail for a number of reasons, but currently no
specific
reason is displayed to the user, e.g.:

virsh -c XenAPI://citrix-xen.example.com/
Enter username for citrix-xen.example.com: root
Enter root's password for citrix-xen.example.com:
error: authentication failed: (null)
error: failed to connect to the hypervisor

This patch displays the session error description on failure.