Check if the buffer is in error state and report an error if it is.

This replaces the pattern:
if (virBufferError(buf)) {
    virReportOOMError();
    goto cleanup;
}
with:

if (virBufferCheckError(buf) < 0)
    goto cleanup;

Document typical buffer usage to favor this.
Also remove the redundant FreeAndReset - if an error has
been set via virBufferSetError, the content is already freed.