arm/xen: fix SMP guests boot

Commit 88e957d6

 ("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>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>