object: Make option help nicer to read

object: Make option help nicer to read

Just like in qemu_opts_print_help(), print the object name as a caption
instead of on every single line, indent all options, add angle brackets
around types, and align the descriptions after 24 characters.

Also, indent every object name in the list of available objects.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>