conf: Don't always recalculate initial memory size from NUMA size totals
    
    When implementing memory hotplug I've opted to recalculate the initial
    memory size (contents of the <memory> element) as a sum of the sizes of
    NUMA nodes when NUMA was enabled. This was based on an assumption that
    qemu did not allow starting when the NUMA node size total didn't equal
    to the initial memory size. Unfortunately the check was introduced to
    qemu just lately.
    
    This patch uses the new XML parser flag to decide whether it's safe to
    update the memory size total from the NUMA cell sizes or not.
    
    As an additional improvement we now report an error in case when the
    size of hotplug memory would exceed the total memory size.
    
    The rest of the changes assures that the function is called with correct
    flags.