xen: increase max number of event channels to 4096

4096 is the maximum possible using the 64-bit standard ABI
(although there is an ABI update which allows more).

This fixes ... Mirage websites which have outrageously large
number of block devices.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
update CHANGES

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
travis: add dependency on xen-evtchn

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
travis: add dependency on xen-gnt

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Merge pull request #91 from djs55/increase-max-event-channels

xen: increase max number of event channels to 4096