xen/intr: fix the event channel enabled per-cpu mask

Fix two issues with the current event channel code, first ENABLED_SETSIZE is
not correctly defined and then using a BITSET to store the per-cpu masks is
not portable to other arches, since on arm32 the event channel arrays shared
with the hypervisor are of type uint64_t and not long. Partially restore the
previous code but switch the bit operations to use the recently introduced
xen_{set/clear/test}_bit versions.

Reviewed by: Julien Grall <julien.grall@citrix.com>
Sponsored by: Citrix Systems R&D
Differential Revision: https://reviews.freebsd.org/D4080