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:
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>