x86/vpt: add support for level interrupts

x86/vpt: add support for level interrupts

Level trigger interrupts will be asserted regardless of whether the
interrupt is masked, and thus the callback will also be executed.

Add a new 'level' parameter to create_periodic_time in order to create
level triggered timers. None of the current users of vpt are switched
to use level triggered interrupts yet.

Note that periodic level triggered interrupts are not supported. This
is because level triggered interrupts always require a deassert of the
IO-APIC pin, which should be done by the caller of vpt at which point
the caller should also reset the timer if required.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>