KVM: selftests: aarch64: Add preemption tests in vgic_irq

Add tests for IRQ preemption (having more than one activated IRQ at the
same time).  This test injects multiple concurrent IRQs and handles them
without handling the actual exceptions.  This is done by masking
interrupts for the whole test.

Signed-off-by: Ricardo Koller <ricarkol@google.com>
Acked-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20211109023906.1091208-11-ricarkol@google.com