conf: Add support for parsing and formatting max memory and slot count

conf: Add support for parsing and formatting max memory and slot count

Add a XML element that will allow to specify maximum supportable memory
and the count of memory slots to use with memory hotplug.

To avoid possible confusion and misuse of the new element this patch
also explicitly forbids the use of the maxMemory setting in individual
drivers's post parse callbacks. This limitation will be lifted when the
support is implemented.