util: virExec may blocked by reading pipe if grandchild prematurely exit

util: virExec may blocked by reading pipe if grandchild prematurely exit

When VIR_EXEC_DAEMON is set, if virPidFileAcquirePath/virSetInherit failed,
then pipesync[0] can not be closed when granchild process exit, because
pipesync[1] still opened in child process. and then saferead in child
process may blocked forever, and left grandchild process in defunct state.

Signed-off-by: Xu Chao <xu.chao6@zte.com.cn>
Signed-off-by: Yi Wang <wang.yi59@zte.com.cn>
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>