qemu: don't append -chardev arg until after value is formatted

qemu: don't append -chardev arg until after value is formatted

The act of formatting a chardev backend value may need to
append command line arguments for passing FDs. If we append
the -chardev arg before formatting the value, then the
resulting arguments will end up interspersed

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>