xen/arm: read ACTLR on the pcpu where the vcpu will run

xen/arm: read ACTLR on the pcpu where the vcpu will run

On big.LITTLE systems not all cores have the same ACTLR. Instead of
reading ACTLR and setting v->arch.actlr in vcpu_initialise, do it later
on the same pcpu where the vcpu will run.

This way, assuming that the vcpu has been created with the right pcpu
affinity, the guest will be able to read the right ACTLR value, matching
the one of the physical cpu.

Also move processor_vcpu_initialise(v) to continue_new_vcpu as it
can modify v->arch.actlr.

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