KVM: nVMX: Use '-1' in 'hv_evmcs_vmptr' to indicate that eVMCS is not in use

Instead of checking 'vmx->nested.hv_evmcs' use '-1' in
'vmx->nested.hv_evmcs_vmptr' to indicate 'evmcs is not in use' state. This
matches how we check 'vmx->nested.current_vmptr'. Introduce EVMPTR_INVALID
and evmptr_is_valid() and use it instead of raw '-1' check as a preparation
to adding other 'special' values.

No functional change intended.

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20210526132026.270394-2-vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>