conf: Replace access to def->mem.max_balloon with accessor functions

conf: Replace access to def->mem.max_balloon with accessor functions

As there are two possible approaches to define a domain's memory size -
one used with legacy, non-NUMA VMs configured in the <memory> element
and per-node based approach on NUMA machines - the user needs to make
sure that both are specified correctly in the NUMA case.

To avoid this burden on the user I'd like to replace the NUMA case with
automatic totaling of the memory size. To achieve this I need to replace
direct access to the virDomainMemtune's 'max_balloon' field with
two separate getters depending on the desired size.

The two sizes are needed as:
1) Startup memory size doesn't include memory modules in some
hypervisors.
2) After startup these count as the usable memory size.

Note that the comments for the functions are future aware and document
state that will be present after a few later patches.