x86/HPET: don't needlessly set up channels for broadcast

x86/HPET: don't needlessly set up channels for broadcast

When there are more FSB delivery capable HPET channels than CPU cores
(or threads), we can simply use a dedicated channel per CPU. This
avoids wasting the resources to handle the excess channels (including
the pointless triggering of the respective interrupt on each
wraparound) as well as the ping-pong of the interrupts' affinities
(when getting assigned to different CPUs).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Keir Fraser <keir@xen.org>