Generic Virtio Device: Upstream xen toolstack support

Sent three versions of the patchset based on review comments and finally
everything is merged now in Xen upstream.

A bug was found on 32 bit platforms with my patchset, invested some time to
get the 32 bit build working on my system, the patch merged without that and
never finished that.

Closing the card now.

Reworked the patches based on reviews and sent back to the list.

Moved to the latest Xen source and found the xen-vhost-frontend broken.

Debugged it and found the issue, fixed it now. The notification mechanism was
changed for the xenstore events.

Finalized the Xen patches for generic virtio device and sent them over to the
list.

Continued working onto this, though couldn't spend a lot of time into this.

Continued working on a generic virtio device support in Xen, which was
suggested by maintainers.

It isn't particularly straight forward as it changes the xenstore interface,
like /local/domain/0/backend/i2c/1/0 becomes
/local/domain/0/backend/virtio/1/0 now and we need to handle it differently
now in the xenstore crate.