when connecting to a GlobalProtect portal (not gateway), generate an xmlconfig so...

when connecting to a GlobalProtect portal (not gateway), generate an xmlconfig so that NetworkManager can list all the gateway servers

GlobalProtect distinguishes "portal" and "gateway" servers.  Often the same
server supports both (/global-protect URLs are for the portal, /ssl-vpn URLs
are for the gateway).  The official clients always connect through the
portal.  Mostly, the portal configuration is not useful for OpenConnect; it
restricts the behavior of the official clients.

However, the portal configuration does contain a list of allowed gateways
(just as AnyConnect VPNs can list other servers).

This commit generates an xmlconfig in the same format as AnyConnect VPNs, so
that the NetworkManager plugins can list all the supported gateways.

Signed-off-by: Daniel Lenski <dlenski@gmail.com>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>