KVM: arm64: Consolidate sysreg userspace accesses

Until now, the .set_user and .get_user callbacks have to implement
(directly or not) the userspace memory accesses. Although this gives
us maximem flexibility, this is also a maintenance burden, making it
hard to audit, and I'd feel much better if it was all located in
a single place.

So let's do just that, simplifying most of the function signatures
in the process (the callbacks are now only concerned with the
data itself, and not with userspace).

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