virfile: Fix error path for forked virFileRemove

virfile: Fix error path for forked virFileRemove

As it turns out the caller in this case expects a return < 0 for failure
and to get/use "errno" rather than using the negative of returned status.
Again different than the create path.

If someone "deleted" a file from the pool without using virsh vol-delete,
then the unlink/rmdir would return an error (-1) and set errno to ENOENT.
The caller checks errno for ENOENT when determining whether to throw an
error message indicating the failure.  Without the change, the error
message is:

error: Failed to delete vol $vol
error: cannot unlink file '/$pathto/$vol': Success

This patch thus allows the fork path to follow the non-fork path
where unlink/rmdir return -1 and errno.

(cherry picked from commit cb19cff468432e55366014658f405066ce06c2f2)