net: Improve error message for -net hubport a bit

Type "hubport" is valid only with -netdev.  Unfortunately, that's
detected late and the error message doesn't explain why:

    $ qemu-system-i386 -net hubport,id=foo,hubid=0
    qemu-system-i386: -net hubport,id=foo,hubid=0: Device 'hubport' could not be initialized

Improve the error message to "Parameter 'type' expects a net type".

Not fixed: -net hubport without the parameters required by -netdev
hubport still asks for those parameters:

    $ qemu-system-i386 -net hubport
    qemu-system-i386: -net hubport: Parameter 'hubid' is missing

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 1431691143-1015-2-git-send-email-armbru@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
