Make xenapi capabilities['enabled'] use service enabled

filters.compute_filter is enabled by default along with libvirt, but the
capabilities['enabled'] section of it is only supported by XenAPI.
This used to work by adding a parameter to capabilities, but since
the capabilities RPC is being removed, use conductor.service_update to tell
the scheduler which host is disabled.

service_update is already the way to tell the scheduler about other
disabled hosts, so to keep backwards compatibility keep host_enabled but
use service_update underneath.

Part of bp no-compute-fanout-to-scheduler

Change-Id: Ica1919cfe9a2402b2107c557c10d47e01a6c430b
Merge "Make xenapi capabilities['enabled'] use service enabled"