xen/arm: vgic: Sanitize target mask used to send SGI

xen/arm: vgic: Sanitize target mask used to send SGI

The current function vgic_to_sgi does not sanitize the target mask and
may therefore get an invalid vCPU ID. This will result to an out of
bound access of d->vcpu[...] as there is no check whether the vCPU ID is
within the maximum supported by the guest.

This was introduced by commit ea37fd2111 "xen/arm: split vgic driver
into generic and vgic-v2 driver".

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