net: fix error reporting for some net parameter checks

A small bit of confusion between buffers is causing errors like:

  qemu: invalid parameter '10' in 'script=/etc/qemu-ifup,fd=10'

instead of:

  qemu: invalid parameter 'script' in 'script=/etc/qemu-ifup,fd=10'

Signed-off-by: Mark McLoughlin <markmc@redhat.com>
