KVM: arm64: vgic-v3: Make the userspace accessors use sysreg API

The vgic-v3 sysreg accessors have been ignored as the rest of the
sysreg internal API was evolving, and are stuck with the .access
method (which is normally reserved to the guest's own access)
for the userspace accesses (which should use the .set/.get_user()
methods).

Catch up with the program and repaint all the accessors so that
they fit into the normal userspace model, and plug the result into
the helpers that have been introduced earlier.

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