xen/arm: Misc improvements to do_common_cpu_on()

xen/arm: Misc improvements to do_common_cpu_on()

 * Use domain_vcpu() rather than opencoding the lookup.  Amongst other things,
   domain_vcpu() is spectre-v1-safe.
 * Unlock the domain immediately after arch_set_info_guest() completes.  There
   is no need for free_vcpu_guest_context() to be within the critical region,
   and moving the call simplifies the error case.

No practical change in functionality.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Julien Grall <julien.grall@arm.com>