intc/arm_gic: Implement virtualization extensions in gic_(activate_irq|drop_prio)

intc/arm_gic: Implement virtualization extensions in gic_(activate_irq|drop_prio)

Implement virtualization extensions in gic_activate_irq() and
gic_drop_prio() and in gic_get_prio_from_apr_bits() called by
gic_drop_prio().

When the current CPU is a vCPU:
  - Use GIC_VIRT_MIN_BPR and GIC_VIRT_NR_APRS instead of their non-virt
  counterparts,
  - the vCPU APR is stored in the virtual interface, in h_apr.

Signed-off-by: Luc Michel <luc.michel@greensocs.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20180727095421.386-11-luc.michel@greensocs.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>