KVM: arm64: nv: Introduce nested virtualization VCPU feature

KVM: arm64: nv: Introduce nested virtualization VCPU feature

Introduce the feature bit and a primitive that checks if the feature is
set behind a static key check based on the cpus_have_const_cap check.

Checking vcpu_has_nv() on systems without nested virt enabled
should have negligible overhead.

We don't yet allow userspace to actually set this feature.

Reviewed-by: Ganapatrao Kulkarni <gankulkarni@os.amperecomputing.com>
Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Christoffer Dall <christoffer.dall@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20230209175820.1939006-4-maz@kernel.org
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>