KVM: arm64: Move vcpu ON_UNSUPPORTED_CPU flag to the state flag set

The ON_UNSUPPORTED_CPU flag is only there to track the sad fact
that we have ended-up on a CPU where we cannot really run.

Since this is only for the host kernel's use, move it to the state
set.

Reviewed-by: Fuad Tabba <tabba@google.com>
Reviewed-by: Reiji Watanabe <reijiw@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>