Ignore missing files on pool refresh

Ignore missing files on pool refresh

If we cannot stat/open a file on pool refresh, returning -1 aborts
the refresh and the pool is undefined.

Only treat missing files as fatal unless VolOpenCheckMode is called
with the VIR_STORAGE_VOL_OPEN_ERROR flag. If this flag is missing
(when it's called from virStorageBackendProbeTarget in
virStorageBackendFileSystemRefresh), only emit a warning and return
-2 to let the caller skip over the file.

https://bugzilla.redhat.com/show_bug.cgi?id=977706