KVM: arm64: Force SCTLR_EL2.WXN when running nVHE

As the EL2 nVHE object is nicely split into sections and that
we already use differenciating permissions for data and code,
we can enable SCTLR_EL2.WXN so that we don't have to worry
about misconfiguration of the page tables.

Flip the WXN bit and get the ball running!

Acked-by: Will Deacon <will@kernel.org>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>