Ignore char devices in storage pools by default

Without this, using /dev/mapper as a directory pool
fails in virStorageBackendUpdateVolTargetInfoFD:
cannot seek to end of file '/dev/mapper/control': Illegal seek

Skip over character devices by default.

https://bugzilla.redhat.com/show_bug.cgi?id=710866

src/storage/storage_backend.h

@@ -130,7 +130,6 @@ enum {
 
 # define VIR_STORAGE_VOL_OPEN_DEFAULT (VIR_STORAGE_VOL_OPEN_ERROR    |\
                                        VIR_STORAGE_VOL_OPEN_REG      |\
-                                       VIR_STORAGE_VOL_OPEN_CHAR     |\
                                        VIR_STORAGE_VOL_OPEN_BLOCK)
 
 int virStorageBackendVolOpenCheckMode(const char *path, struct stat *sb,