KVM: arm64: Add helper to identify a nested context

A common idiom in the KVM code is to check if we are currently
dealing with a "nested" context, defined as having NV enabled,
but being in the EL1&0 translation regime.

This is usually expressed as:

	if (vcpu_has_nv(vcpu) && !is_hyp_ctxt(vcpu) ... )

which is a mouthful and a bit hard to read, specially when followed
by additional conditions.

Introduce a new helper that encapsulate these two terms, allowing
the above to be written as

	if (is_nested_context(vcpu) ... )

which is both shorter and easier to read, and makes more obvious
the potential for simplification on some code paths.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20250708172532.1699409-4-oliver.upton@linux.dev


Signed-off-by: Oliver Upton <oliver.upton@linux.dev>