virbuf: more detailed error reporting

virbuf: more detailed error reporting

The next patch wants to add some sanity checking, which would
be a different error than ENOMEM.  Many existing callers blindly
report OOM failure if virBuf reports an error, and this will be
wrong in the (unlikely) case that they actually had a usage error
instead; but since the most common error really is ENOMEM, I'm
not going to fix all callers.  Meanwhile, new discriminating
callers can react differently depending on what failure happened.

* src/util/buf.c (virBufferSetError): Add parameter.
(virBufferGrow, virBufferVasprintf, virBufferEscapeString)
(virBufferEscapeSexpr, virBufferEscapeShell): Adjust callers.