qemu: don't use chardev FD passing for vhostuser backend

qemu: don't use chardev FD passing for vhostuser backend

QEMU chardevs have a bug which makes the vhostuser backend complain
about lack of support for FD passing when validating the chardev.
While this is ultimately QEMU's responsibility to fix, libvirt needs to
avoid tickling the bug.

Simply disabling chardev FD passing just for vhostuser's chardev is
the most prudent approach, avoiding need for a QEMU version number
check.

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