arm/xen: fix SMP guests boot

arm/xen: fix SMP guests boot

Commit 88e957d6e47f ("xen: introduce xen_vcpu_id mapping") broke SMP ARM
guests on Xen. When FIFO-based event channels are in use (this is the
default), evtchn_fifo_alloc_control_block() is called on CPU_UP_PREPARE
event and this happens before we set up xen_vcpu_id mapping in
xen_starting_cpu. Temporary fix the issue by setting direct Linux CPU id
<-> Xen vCPU id mapping for all possible CPUs at boot. We don't currently
support kexec/kdump on Xen/ARM so these ids always match.

In future, we have several ways to solve the issue, e.g.:
- Eliminate all hypercalls from CPU_UP_PREPARE, do them from the starting
CPU. This can probably be done for both x86 and ARM and, if done, will
allow us to get Xen's idea of vCPU id from CPUID/MPIDR on the starting CPU
directly, no messing with ACPI/device tree required.
- Save vCPU id information from ACPI/device tree on ARM and use it to
initialize xen_vcpu_id mapping. This is the same trick we currently do on
x86.

Reported-by: Julien Grall <julien.grall@arm.com>
Tested-by: Wei Chen <Wei.Chen@arm.com>
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>