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

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

Commit 43f187a 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 43f187a, and strip
them in hxtool instead.

Cc: Paolo Bonzini <[email protected]>
CC: [email protected]
Signed-off-by: Markus Armbruster <[email protected]>
Message-Id: <20171002140307[email protected]>
Reviewed-by: Marc-André Lureau <[email protected]>