conf: allow fuzz in XML with cur balloon > max
    
    Commit 1b1402b introduced a regression.  Since older libvirt versions
    would silently round memory up (until the previous patch), but populated
    current memory based on querying the guest, it was possible to have
    dumpxml show cur > max by the amount of the rounding.  For example, if
    a user requested 1048570 KiB memory (just shy of 1GiB), the qemu
    driver would actually run with 1048576 KiB, and libvirt 0.9.10 would
    output a current that was 6KiB larger than the maximum.  Situations
    where this could have an impact include, but are not limited to,
    migration from old to new libvirt, managedsave in old libvirt and
    start in new libvirt, snapshot creation in old libvirt and revert in
    new libvirt - without this patch, the new libvirt would reject the
    VM because of the rounding discrepancy.
    
    Fix things by adding a fuzz factor, and silently clamp current down to
    maximum in that case, rather than failing to reparse XML for an existing
    VM.  From a practical standpoint, this has no user impact: 'virsh
    dumpxml' will continue to query the running guest rather than rely on
    the incoming xml, which will see the currect current value, and even if
    clamping down occurs during parsing, it will be by at most the fuzz
    factor of a megabyte alignment, and rounded back up when passed back to
    the hypervisor.
    
    Meanwhile, we continue to reject cur > max if the difference is beyond
    the fuzz factor of nearest megabyte.  But this is not a real change in
    behavior, since with 0.9.10, even though the parser allowed it, later
    in the processing stream we would reject it at the qemu layer; so
    rejecting it in the parser just moves error detection to a nicer place.
    
    * src/conf/domain_conf.c (virDomainDefParseXML): Don't reject
    existing XML.
    Based on a report by Zhou Peng.