fsdev: Error out when unsupported option is passed

Each fsdriver only supports a subset of the options that can be passed
to -fsdev. Unsupported options are simply ignored. This could cause the
user to erroneously think QEMU has a bug.

Enforce strict checking of supported options for all fsdrivers. This
shouldn't impact libvirt, since it doesn't know about the synth and
proxy fsdrivers.

Signed-off-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Eric Blake <eblake@redhat.com>