xen/arm: make domain_max_vcpus return value from vgic_ops

xen/arm: make domain_max_vcpus return value from vgic_ops

Each vGIC driver supports different maximum numbers of vCPU. For
example, GICv2 is limited to 8 vCPUs, while GICv3 can support up
to 4096 vCPUs if we use both AFF0 and AFF1. Thus, domain_max_vcpus
should depend on not only MAX_VIRT_CPUS but also the version
of vGIC that the guest uses.

Since evtchn_init would call domain_max_vcpus to allocate poll_mask
when the vgic_ops haven't been initialised yet, we make it return
MAX_VIRT_CPUS at that time. On ARM32, event channel doesn't need
to allocate the poll_mask because MAX_VIRT_CPUS < BITS_PER_LONG,
while allocating more memory (2 unsigned long rather than 1) only
for poll_mask on arm64 with GICv2 looks not so expensive.

We didn't keep it as the old static inline form because it will break
compilation when access the member of struct domain:

In file included from xen/include/xen/domain.h:6:0,
                 from xen/include/xen/sched.h:10,
                 from arm64/asm-offsets.c:10:
xen/include/asm/domain.h: In function ‘domain_max_vcpus’:
xen/include/asm/domain.h:266:10: error: dereferencing pointer to incomplete type
     if (d->arch.vgic.version == GIC_V2)
          ^

Signed-off-by: Chen Baozi <baozich@gmail.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>