block/xen: Use blk_new_open() in blk_connect()

As part of the required changes, this fixes a bug where specifying an
invalid driver would result in the block layer probing the image format;
now it will result in an error, unless "<unset>" is specified as the
driver name. Fixing this would require further work on the xen_disk code
which does not seem worth it (at this point and for this patch).

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 1423162705-32065-7-git-send-email-mreitz@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
