Bugzilla: session parameters for XML RPC calls

The session parameters are added to every XMLRPC call.

abrt_xmlrpc_call*() functions expected formatting string in form of
"({...})" for some good but unknown reason. Since now, the functions
expects formatting string without the outer brackets.

() - means empty array (allowed in xmlrpc-c)
{} - means empty structure (allowed in xmlrpc-c)

Signed-off-by: Jakub Filak <jfilak@redhat.com>