Update git submodules

* Update docs/submodules/releng-xci from branch 'master'
  - xci: Switch VM disk cache to 'unsafe' and use 'iothreads' for I/O
    
    According to the docs[1]
    
    "writeback: This mode causes the hypervisor to interact with the disk
    image file or block device with neither O_DSYNC nor O_DIRECT semantics.
    The host page cache is used and writes are reported to the guest as
    completed when they are placed in the host page cache. The normal page
    cache management will handle commitment to the storage device.
    Additionally, the guest's virtual storage adapter is informed of the
    writeback cache, so the guest would be expected to send down flush
    commands as needed to manage data integrity. Analogous to a raid
    controller with RAM cache."
    
    and
    
    "writeback: This mode informs the guest of the presence of a write
    cache, and relies on the guest to send flush commands as needed to
    maintain data integrity within its disk image. This is a common
    storage design which is completely accounted for within modern file
    systems. This mode exposes the guest to data loss in the unlikely case
    of a host failure, because there is a window of time between the time
    a write is reported as completed, and that write being committed to the
    storage device."
    
    "unsafe: This mode is similar to writeback caching except for the
    following: the guest flush commands are ignored, nullifying the data
    integrity control of these flush commands, and resulting in a higher
    risk of data loss because of host failure. The name “unsafe” should
    serve as a warning that there is a much higher potential for data
    loss because of a host failure than with the other modes. As the
    guest terminates, the cached data is flushed at that time."
    
    It's beneficial to use the host page cache to cache I/O from the guest
    instead of waiting for data to reach the actual disk device. We do not
    normally care about data integrity so data loss is not a problem.
    
    Moreover, we drop the cache configuration from the flavor files since
    it's independent of the flavor that's being deployed.
    
    [1] https://www.suse.com/documentation/sles-12/singlehtml/book_virt/book_virt.html#cha.cachemodes
    
    Change-Id: I118ffdf84b1be672185b3eff60fe5d0b5f1a590d
    Signed-off-by: Markos Chandras <mchandras@suse.de>
    

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