While we still don't want to enable gcc's new -Wformat-literal
warning, I found a rather easy case where the warning could be
reduced, by getting rid of obsolete error-reporting practices.
This is the last place where we were passing the (unused) net
and conn arguments for constructing an error.

* src/util/virterror_internal.h (virErrorMsg): Delete prototype.
(virReportError): Delete macro.
* src/util/virterror.c (virErrorMsg): Make static.
* src/libvirt_private.syms (virterror_internal.h): Drop export.
* src/util/conf.c (virConfError): Convert to macro.
(virConfErrorHelper): New function, and adjust error calls.
* src/xen/xen_hypervisor.c (virXenErrorFunc): Delete.
(xenHypervisorGetSchedulerType)
(xenHypervisorGetSchedulerParameters)
(xenHypervisorSetSchedulerParameters)
(xenHypervisorDomainBlockStats)
(xenHypervisorDomainInterfaceStats)
(xenHypervisorDomainGetOSType)
(xenHypervisorNodeGetCellsFreeMemory, xenHypervisorGetVcpus):
Update callers.