target-i386: Make "level" and "xlevel" properties static

Static properties require only 1 line of code, much simpler than the
existing code that requires writing new getters/setters.

As a nice side-effect, this fixes an existing bug where the setters were
incorrectly allowing the properties to be changed after the CPU was
already realized.

Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>