ARM: evtchn: Handle level triggered IRQs correctly

ARM: evtchn: Handle level triggered IRQs correctly

The event channel IRQ has level triggered semantics, however the current
VGIC treats everything as edge triggered.
To correctly process those IRQs, we have to lower the (virtual) IRQ line
at some point in time, depending on whether the interrupt condition
still prevails.
Check the per-VCPU evtchn_upcall_pending variable to make the interrupt
line match its status, and call this function upon every hypervisor
entry.

Signed-off-by: Andre Przywara <andre.przywara@linaro.org>
Reviewed-by: Julien Grall <julien.grall@arm.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>