storage: conf: Correctly calculate exabyte unit

We were using 'Y' to mean exabyte, when the correct abbreviation would be
'E' ('Y' is yettabyte, which is exabyte * 1024 * 1024). While it isn't
strictly backwards compatible, I highly doubt anyone was actually using
this broken behavior, so I don't see any harm in in dropping 'Y' handling.

src/conf/storage_conf.c

@@ -980,8 +980,8 @@ virStorageSize(const char *unit,
             mult = 1024ull * 1024ull * 1024ull * 1024ull * 1024ull;
             break;
 
-        case 'y':
-        case 'Y':
+        case 'e':
+        case 'E':
             mult = 1024ull * 1024ull * 1024ull * 1024ull * 1024ull *
                 1024ull;
             break;