xen/arm: set VPIDR based on the MIDR value of the underlying pCPU

xen/arm: set VPIDR based on the MIDR value of the underlying pCPU

On big.LITTLE systems not all cores have the same MIDR. Instead of
storing only one VPIDR per domain, initialize it to the value of the
MIDR of the 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 VPIDR value, matching
the one of the physical cpu.

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