vl: introduce object_parse_property_opt

vl: introduce object_parse_property_opt

We will reuse the parsing loop of machine_set_property soon for "-accel",
but we do not want the "_" -> "-" conversion since "-accel" can just
standardize on dashes.  We will also add a bunch of legacy option handling
to keep the QOM machine object clean.  Extract the loop into a separate
function, and keep the legacy handling in machine_set_property.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>