net: raise an error if -net type is invalid

When a -net type is used that was not compiled into the binary there
should be an error message.

Note the special case for -net none, which is a no-op.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-id: 1432743412-15943-4-git-send-email-stefanha@redhat.com