virtfs: error out gracefully when mandatory suboptions are missing

We internally convert -virtfs to -fsdev/-device. If the user doesn't
provide the path or security_model suboptions, and the fsdev backend
requires them, we hit an assertion when populating the internal -fsdev
option:

util/qemu-option.c:547: opt_set: Assertion `opt->str' failed.
Aborted (core dumped)

Let's test the suboption presence on the command line before trying
to set it in the internal -fsdev option, and let the backend code
error out gracefully (ie, like it already does when the user passes
-fsdev on the command line).

Reported-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
(cherry picked from commit 32b69436)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>

vl.c

@@ -3503,7 +3503,7 @@ int main(int argc, char **argv, char **envp)
             case QEMU_OPTION_virtfs: {
                 QemuOpts *fsdev;
                 QemuOpts *device;
-                const char *writeout, *sock_fd, *socket;
+                const char *writeout, *sock_fd, *socket, *path, *security_model;
 
                 olist = qemu_find_opts("virtfs");
                 if (!olist) {
@@ -3541,11 +3541,15 @@ int main(int argc, char **argv, char **envp)
                 }
                 qemu_opt_set(fsdev, "fsdriver",
                              qemu_opt_get(opts, "fsdriver"), &error_abort);
-                qemu_opt_set(fsdev, "path", qemu_opt_get(opts, "path"),
-                             &error_abort);
-                qemu_opt_set(fsdev, "security_model",
-                             qemu_opt_get(opts, "security_model"),
+                path = qemu_opt_get(opts, "path");
+                if (path) {
+                    qemu_opt_set(fsdev, "path", path, &error_abort);
+                }
+                security_model = qemu_opt_get(opts, "security_model");
+                if (security_model) {
+                    qemu_opt_set(fsdev, "security_model", security_model,
                                  &error_abort);
+                }
                 socket = qemu_opt_get(opts, "socket");
                 if (socket) {
                     qemu_opt_set(fsdev, "socket", socket, &error_abort);