conf: Add a new xenbus controller option for event channels

conf: Add a new xenbus controller option for event channels

Event channels are like PV interrupts and in conjuction with grant frames
form a data transfer mechanism for PV drivers. They are also used for
inter-processor interrupts. Guests with a large number of vcpus and/or
many PV devices many need to increase the maximum default value of 1023.
For this reason the native Xen config format supports the
'max_event_channels' setting. See xl.cfg(5) man page for more details.

Similar to the existing maxGrantFrames option, add a new xenbus controller
option 'maxEventChannels', allowing to adjust the maximum value via libvirt.

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