command: avoid fd leak on failure

command: avoid fd leak on failure

virCommandTransferFD promises that the fd is no longer owned by
the caller.  Normally, we want the fd to remain open until the
child runs, but in error situations, we must close it earlier.

* src/util/command.c (virCommandTransferFD): Close fd now if we
can't track it to close later.
(virCommandKeepFD): Adjust helper to make this easier.