errors: don't make GetLastError() segfault when on no error condition

[vincentbernat]

When calling GetLastError() twice in a row, we get a segfault because virGetLastError() C function returns nil. We cannot return nil since we don't return a pointer, but we can return a special error that means "no error".

[rgbkrk]

Rebase this and let's merge it.

[vincentbernat]

Rebased too.