xmlrpc: marshalling of Long type

The change let people use 64bit numerical type (long) with xmlrpc.
Jsonrpc already supports long type.

There is xmlrpc extension i8 which let people use long type but we do
not want to enable it due to backward compatibility reasons.

In order to use this code developer needs to add method name to
SUPPORTED_METHODS_FOR_LONG_CONVERSION collection. This reduces overhead
of type fixing only to known methods. Long type is normalized to string
to let older vdsms understand the value.


Change-Id: I97ffc2da3703dbf7ce58607b7461a02aea35f2bb
Signed-off-by: pkliczewski <piotr.kliczewski@gmail.com>