remove trailing whitespace from qemu-options.hx

Remove trailing whitespace in qemu-options documentation, as it causes
reproducibility issues depending on the echo implementation used by
the Makefile.
Reported-By: NVagrant Cascadian <vagrant@debian.org>
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>
Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>