KVM: arm64: Move HCRX_EL2 switch to load/put on VHE systems

Although the nVHE behaviour requires HCRX_EL2 to be switched
on each switch between host and guest, there is nothing in
this register that would affect a VHE host.

It is thus possible to save/restore this register on load/put
on VHE systems, avoiding unnecessary sysreg access on the hot
path. Additionally, it avoids unnecessary traps when running
with NV.

To achieve this, simply move the read/writes to the *_common()
helpers, which are called on load/put on VHE, and more eagerly
on nVHE.

Reviewed-by: Eric Auger <eric.auger@redhat.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Jing Zhang <jingzhangos@google.com>
Link: https://lore.kernel.org/r/20230815183903.2735724-28-maz@kernel.org