xen/arm: traps: Rework leave_hypervisor_tail

xen/arm: traps: Rework leave_hypervisor_tail

The function leave_hypervisor_tail is called before each return to the
guest vCPU. It has two main purposes:
    1) Process physical CPU work (e.g rescheduling) if required
    2) Prepare the physical CPU to run the guest vCPU

2) will always be done once we finished to process physical CPU work. At
the moment, it is done part of the last iterations of 1) making adding
some extra indentation in the code.

This could be streamlined by moving out 2) of the loop. At the same
time, 1) is moved in a separate function making more obvious what is
happening.

All those changes will help a follow-up patch where we would want to
introduce some vCPU work before returning to the guest vCPU.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>