virSocketSendMsgWithFDs: Don't report errors, just set errno

virSocketSendMsgWithFDs: Don't report errors, just set errno

Currently, virSocketSendMsgWithFDs() reports two errors:

1) if CMSG_FIRSTHDR() fails,
2) if sendmsg() fails.

Well, the latter sets an errno, so caller can just use
virReportSystemError(). And the former - it is very unlikely to
fail because memory for whole control message was allocated just
a few lines above.

The motivation is to unify behavior of virSocketSendMsgWithFDs()
and virSocketSendFD() because the latter is just a subset of the
former (will be addressed later).

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>