Bug 971959

Upstream still has some sites using the old QERR_OPEN_FILE_FAILED macro. This hurts error messages as it doesn't include the error reason. Also, RHEL6 got a workaround for this, so we ought to fix this upstream & RHEL7.

To get a listing of the call sites, just do:

$ git grep QERR_OPEN_FILE_FAILED
backends/rng-random.c:            error_set(errp, QERR_OPEN_FILE_FAILED, s->filename)
block.c:                error_set(errp, QERR_OPEN_FILE_FAILED,
block/mirror.c:        error_set(errp, QERR_OPEN_FILE_FAILED, backing_filename);
blockdev.c:        error_set(errp, QERR_OPEN_FILE_FAILED, new_image_file);
blockdev.c:        error_set(errp, QERR_OPEN_FILE_FAILED, filename);
blockdev.c:        error_set(errp, QERR_OPEN_FILE_FAILED, target);
cpus.c:        error_set(errp, QERR_OPEN_FILE_FAILED, filename);
cpus.c:        error_set(errp, QERR_OPEN_FILE_FAILED, filename);
dump.c:            error_set(errp, QERR_OPEN_FILE_FAILED, p);
include/qapi/qmp/qerror.h:#define QERR_OPEN_FILE_FAILED \
savevm.c:        error_set(errp, QERR_OPEN_FILE_FAILED, filename);

The fix is simple: just convert those calls to error_setg_errno().