KVM: selftests: aarch64: Add ISPENDR write tests in vgic_irq

Add injection tests that use writing into the ISPENDR register (to mark
IRQs as pending). This is typically used by migration code.

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/20211109023906.1091208-17-ricarkol@google.com