vl: introduce machine_merge_property

It will be used to parse smp-opts config groups from configuration
files.  The point to note is that it does not steal a reference
from the caller.  This is better because this function will be called
from qemu_config_foreach's callback; qemu_config_foreach does not cede
its reference to the qdict to the callback, and wants to free it.  To
balance that extra reference, machine_parse_property_opt now needs
a qobject_unref.

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