xenconfig: Add support for max_event_channels

Add support in the domXML<->native config converter for max_event_channels.
The parser and formater functions for max_grant_frames were reworked to
also parse max_event_channels. In doing so the xenbus controller is added
earlier in the config parsing, requiring a small adjustment to one of the
existing tests. Include a new test for the event channel conversion.

Signed-off-by: Jim Fehlig <jfehlig@suse.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>