docs: Fix generating qemu-doc.html with texinfo 5

LC_ALL=C makeinfo --no-headers --no-split --number-sections --html qemu-doc.texi -o qemu-doc.html
./qemu-options.texi:1521: unknown command `list'
./qemu-options.texi:1521: table requires an argument: the formatter for @item
./qemu-options.texi:1521: warning: @table has text but no @item

This is for 1.4 stable only; master isn't affected, as it was fixed by
another commit (which isn't appropriate for stable):

commit 5d6768e3b8908a60f0a3016b7fa24194f6b47c80
Author: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
Date:   Fri Feb 22 12:39:51 2013 +0900

    sheepdog: accept URIs

Signed-off-by: Cole Robinson <crobinso@redhat.com>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>

qemu-options.hx

@@ -2092,18 +2092,13 @@ QEMU supports using either local sheepdog devices or remote networked
 devices.
 
 Syntax for specifying a sheepdog device
-@table @list
-``sheepdog:<vdiname>''
-
-``sheepdog:<vdiname>:<snapid>''
-
-``sheepdog:<vdiname>:<tag>''
-
-``sheepdog:<host>:<port>:<vdiname>''
-
-``sheepdog:<host>:<port>:<vdiname>:<snapid>''
-
-``sheepdog:<host>:<port>:<vdiname>:<tag>''
+@table @code
+@item sheepdog:<vdiname>
+@item sheepdog:<vdiname>:<snapid>
+@item sheepdog:<vdiname>:<tag>
+@item sheepdog:<host>:<port>:<vdiname>
+@item sheepdog:<host>:<port>:<vdiname>:<snapid>
+@item sheepdog:<host>:<port>:<vdiname>:<tag>
 @end table
 
 Example