Allow disabling the tx checksums on the xen loopback

Allow disabling the tx checksums on the xen loopback
interface, which is used for the virtual interfaces of dom0 instead
of netfront/netback.

I need to issue ethtool -K eth0 tx off to work around a bad checksum
problem, which has been reported for domUs a few months ago. I'm
bridging dom0's eth0 to a domU where I do NAT to a different virtual
interface. Without this change, I get lots of packets with bad
checksums.

Signed-off-by: Jan Niehusmann <jan@gondor.com>