xen/arm: vgic: Rename nr_lines into nr_spis

xen/arm: vgic: Rename nr_lines into nr_spis

The field nr_lines in the arch_domain vgic structure contains the number of
SPIs for the emulated GIC. Using the nr_lines make confusion with the GIC
code, where it means the number of IRQs. This can lead to coding error.

Also introduce vgic_num_irqs to get the number of IRQ handled by the emulated
GIC.

Finally drop the initialization of nr_spis in both gicv2v_init and gicv3_init
as it's already initialized in vgic common code.

Signed-off-by: Julien Grall <julien.grall@linaro.org>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>