qemu-options: Remove stray colons from output of --help

Commit 43f187a5 broke --help: it put colons into blank lines.  It
removed the colon from DEFHEADING(TITLE:) and added it back in the
macro expansion of DEFHEADING(TITLE), so hxtool can emit "@subsection
TITLE" more easily.  Trouble is it's added back even for the blank
lines made with DEFHEADING().

Put the colons back where they were before commit 43f187a5, and strip
them in hxtool instead.

Cc: Paolo Bonzini <pbonzini@redhat.com>
CC: qemu-stable@nongnu.org
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20171002140307.5292-2-armbru@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>