qom: Factor out user_creatable_process_cmdline()

qom: Factor out user_creatable_process_cmdline()

The implementation for --object can be shared between
qemu-storage-daemon and other binaries, so move it into a function in
qom/object_interfaces.c that is accessible from everywhere.

This also requires moving the implementation of qmp_object_add() into a
new user_creatable_add_qapi(), because qom/qom-qmp-cmds.c is not linked
for tools.

user_creatable_print_help_from_qdict() can become static now.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Acked-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>