KVM: arm64: vgic-v3: Push user access into vgic_v3_cpu_sysregs_uaccess()

In order to start making the vgic sysreg access from userspace
similar to all the other sysregs, push the userspace memory
access one level down into vgic_v3_cpu_sysregs_uaccess().

The next step will be to rely on the sysreg infrastructure
to perform this task.

Reviewed-by: Reiji Watanabe <reijiw@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>