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>