KVM: selftests: aarch64: Level-sensitive interrupts tests in vgic_irq

Add a cmdline arg for using level-sensitive interrupts (vs the default
edge-triggered). Then move the handler into a generic handler function
that takes the type of interrupt (level vs. edge) as an arg.  When
handling line-sensitive interrupts it sets the line to low after
acknowledging the IRQ.

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/20211109023906.1091208-12-ricarkol@google.com