Fix usage of virReportSystemError

virReportSystemError is reserved for reporting system errors, calling it
with VIR_ERR_* error codes produces error messages that do not make any
sense, such as

    internal error: guest failed to start: Kernel doesn't support user
    namespace: Link has been severed

We should prohibit wrong usage with a syntax-check rule.

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>