rpc: message related sizes enlarged

rpc: message related sizes enlarged

We have seen an issue on s390x platform where domain XMLs larger than 1MB
were used. The define command was finished successfully. The dumpxml command
was not successful (i.e. could not encode message payload).

Enlarged message related sizes (e.g. maximum string size, message size, etc.)
to handle larger system configurations used on s390x platform.

To improve handling of the RPC message size the allocation during encode process
is changed to a dynamic one (i.e. starting with 64kB initial size and increasing
that size in steps up to 16MB if the payload data is larger).

Signed-off-by: Daniel Hansel <daniel.hansel@linux.vnet.ibm.com>
Signed-off-by: Viktor Mihajlovski <mihajlov@linux.vnet.ibm.com>