kvm: Add fake KVM constants to avoid #ifdefs on KVM-specific code

Any KVM-specific code that use these constants must check if
kvm_enabled() is true before using them.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
