xen/arm: Use AFF1 when translating ICC_SGI1R_EL1 to cpumask

xen/arm: Use AFF1 when translating ICC_SGI1R_EL1 to cpumask

The old unsigned long type of vcpu_mask can only express 64 cpus at the
most, which might not be enough for the guest which used vGICv3. We
introduce a new struct sgi_target for the target cpu list of SGI, which
holds the affinity path information (only level 1 at the moment). For
GICv2 that has no affinity level, we can just set the corresponding
fields to be 0.

Signed-off-by: Chen Baozi <baozich@gmail.com>