intc/arm_gic: Implement gic_update_virt() function

intc/arm_gic: Implement gic_update_virt() function

Add the gic_update_virt() function to update the vCPU interface states
and raise vIRQ and vFIQ as needed. This commit renames gic_update() to
gic_update_internal() and generalizes it to handle both cases, with a
`virt' parameter to track whether we are updating the CPU or vCPU
interfaces.

The main difference between CPU and vCPU is the way we select the best
IRQ. This part has been split into the gic_get_best_(v)irq functions.
For the virt case, the LRs are iterated to find the best candidate.

Signed-off-by: Luc Michel <luc.michel@greensocs.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20180727095421.386-17-luc.michel@greensocs.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>