qdev: allow reusing get/set for legacy property

qdev: allow reusing get/set for legacy property

In some cases, a legacy property does need a special print method
but not a special parse method.  In this case, we can reuse the get/set
from the static (non-legacy) property.

If neither parse nor print is needed, though, do not register the
legacy property at all.  The previous patch ensures that the right
fallback will be used.

Reviewed-by: Anthony Liguori <aliguori@us.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>