Turn libvirt.c error reporting functions into macros

The virLibConnError() function (and related ones) do not correctly
report line number info. Turn them all into macros so line numbers
are reported correctly. Drop the connection object in all of them
since it is no longer used.

Also from the virLibConnWarning() equivalents completely. Now
that the Xen driver is running 100% inside libvirtd, those
codepaths for secondary drivers cannot be reached.

* src/libvirt.c: Replace error functions with macros