kvm tools: Check for unknown parameters in network option handling

kvm tools: Check for unknown parameters in network option handling

Currently the parsing of network command line parameters doesn't check
for unknown parameters.

For example "-n mod=none" will cause kvmtool to setup TAP networking.

Save users from their typos by checking for unknown parameters and bailing.

Acked-by: Asias He <asias.hejun@gmail.com>
Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Pekka Enberg <penberg@kernel.org>