This switches the HMP command object_add from a QemuOpts-based parser to
user_creatable_add_from_str() which uses a keyval parser and enforces
the QAPI schema.

Apart from being a cleanup, this makes non-scalar properties and help
accessible. In order for help to be printed to the monitor instead of
stdout, the printf() calls in the help functions are changed to
qemu_printf().

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>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>