KVM: arm64: Only open the interrupt window on exit due to an interrupt

Now that we properly account for interrupts taken whilst the guest
was running, it becomes obvious that there is no need to open
this accounting window if we didn't exit because of an interrupt.

This saves a number of system register accesses and other barriers
if we exited for any other reason (such as a trap, for example).

Signed-off-by: Marc Zyngier <maz@kernel.org>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Link: https://lore.kernel.org/r/20220304135914.1464721-1-maz@kernel.org