qom: move user_creatable_add_opts logic to vl.c and QAPIfy it

qom: move user_creatable_add_opts logic to vl.c and QAPIfy it

Emulators are currently using OptsVisitor (via user_creatable_add_opts)
to parse the -object command line option.  This has one extra feature,
compared to keyval, which is automatic conversion of integers to lists
as well as support for lists as repeated options:

  -object memory-backend-ram,id=pc.ram,size=1048576000,host-nodes=0,policy=bind

So we cannot replace OptsVisitor with keyval right now.  Still, this
patch moves the user_creatable_add_opts logic to vl.c since it is
not needed anywhere else, and makes it go through user_creatable_add_qapi.

In order to minimize code changes, the predicate still takes a string.
This can be changed later to use the ObjectType QAPI enum directly.

Reviewed-by: Eric Blake <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>
Message-Id: <20210312173547.1283477[email protected]>
Reviewed-by: Markus Armbruster <[email protected]>
Signed-off-by: Kevin Wolf <[email protected]>