another patch related to the [XSA-240, CVE-2017-15595] issue
    xen: various flaws (#1525018)
      x86 PV guests may gain access to internally used page [XSA-248]
      broken x86 shadow mode refcount overflow check [XSA-249]
      improper x86 shadow mode refcount error handling [XSA-250]
      improper bug check in x86 log-dirty handling [XSA-251]