qemu-io: Use purely string blockdev options

Currently, qemu-io only uses string-valued blockdev options (as all are
converted directly from QemuOpts) -- with one exception: -U adds the
force-share option as a boolean.  This in itself is already a bit
questionable, but a real issue is that it also assumes the value already
existing in the options QDict would be a boolean, which is wrong.

That has the following effect:

$ ./qemu-io -r -U --image-opts \
    driver=file,filename=/dev/null,force-share=off
[1]    15200 segmentation fault (core dumped)  ./qemu-io -r -U
--image-opts driver=file,filename=/dev/null,force-share=off

Since @opts is converted from QemuOpts, the value must be a string, and
we have to compare it as such.  Consequently, it makes sense to also set
it as a string instead of a boolean.

Cc: qemu-stable@nongnu.org
Signed-off-by: NMax Reitz <mreitz@redhat.com>
Message-id: 20180502202051.15493-2-mreitz@redhat.com
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NMax Reitz <mreitz@redhat.com>
(cherry picked from commit 2a01c01f)
Signed-off-by: NMichael Roth <mdroth@linux.vnet.ibm.com>

qemu-io.c

@@ -71,12 +71,12 @@ static int openfile(char *name, int flags, bool writethrough, bool force_share,
             opts = qdict_new();
         }
         if (qdict_haskey(opts, BDRV_OPT_FORCE_SHARE)
-            && !qdict_get_bool(opts, BDRV_OPT_FORCE_SHARE)) {
+            && strcmp(qdict_get_str(opts, BDRV_OPT_FORCE_SHARE), "on")) {
             error_report("-U conflicts with image options");
             QDECREF(opts);
             return 1;
         }
-        qdict_put_bool(opts, BDRV_OPT_FORCE_SHARE, true);
+        qdict_put_str(opts, BDRV_OPT_FORCE_SHARE, "on");
     }
     qemuio_blk = blk_new_open(name, NULL, opts, flags, &local_err);
     if (!qemuio_blk) {