x86/xen: Add "xen_timer_slop" command line option

Add a new command-line option "xen_timer_slop=<INT>" that sets the
minimum delta of virtual Xen timers. This commit does not change the
default timer slop value for virtual Xen timers.

Lowering the timer slop value should improve the accuracy of virtual
timers (e.g., better process dispatch latency), but it will likely
increase the number of virtual timer interrupts (relative to the
original slop setting).

The original timer slop value has not changed since the introduction
of the Xen-aware Linux kernel code. This commit provides users an
opportunity to tune timer performance given the refinements to
hardware and the Xen event channel processing. It also mirrors
a feature in the Xen hypervisor - the "timer_slop" Xen command line
option.

[boris: updated comment describing TIMER_SLOP]

Signed-off-by: Ryan Thibodeaux <ryan.thibodeaux@starlab.io>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>