KVM: arm64: Make HCR_EL2 a non-VNCR register

FEAT_NV3 makes a fundamental change to the architecture, by moving
guest-initiated HCR_EL2 accesses to the NVHCR_EL2 register. As the
names suggests, this is HCR_EL2 for a NV guest.

But where do NVHCR_EL2 accesses from a guest go? The are redirected
to the VNCR page, right where HCR_EL2 is stored in the NV2 case.
Does it hurt? Good. There's more coming.

The challenge here is to make KVM work seamlessly, without rewriting
everything. Which implies that things such as __vcpu_sys_reg(HCR_EL2)
must work, no matter the underlying NV implementation.

A simple way to deal with it is to move HCR_EL2's canonical storage
outside of VNCR for the vast majority of the KVM code, and only have
a copy at entry/exit times. Given that we don't really support NV3
yet, this is pretty simple.

In the process, advertise NVHCR_EL2 as the register that now holds
offset 0x78 in the VNCR page.

Reviewed-by: Joey Gouly <joey.gouly@arm.com>
Reviewed-by: Yuan Yao <yaoyuan@linux.alibaba.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://patch.msgid.link/20260730071022.296811-20-maz@kernel.org
Signed-off-by: Oliver Upton <oupton@kernel.org>

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