arm/arm64: xen: Fix to convert percpu address to gfn correctly

Use per_cpu_ptr_to_phys() instead of virt_to_phys() for per-cpu
address conversion.

In xen_starting_cpu(), per-cpu xen_vcpu_info address is converted
to gfn by virt_to_gfn() macro. However, since the virt_to_gfn(v)
assumes the given virtual address is in linear mapped kernel memory
area, it can not convert the per-cpu memory if it is allocated on
vmalloc area.

This depends on CONFIG_NEED_PER_CPU_EMBED_FIRST_CHUNK.
If it is enabled, the first chunk of percpu memory is linear mapped.
In the other case, that is allocated from vmalloc area. Moreover,
if the first chunk of percpu has run out until allocating
xen_vcpu_info, it will be allocated on the 2nd chunk, which is
based on kernel memory or vmalloc memory (depends on
CONFIG_NEED_PER_CPU_KM).

Without this fix and kernel configured to use vmalloc area for
the percpu memory, the Dom0 kernel will fail to boot with following
errors.

[    0.466172] Xen: initializing cpu0
[    0.469601] ---------...