chardev: Indent list of chardevs

Following the example of qemu_opts_print_help(), indent all entries in
the list of character devices.

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>