ram_block: use Number_of_bytes to parse block size

Using this helper rather than parsing as size_t directly allows use to
use size suffixes, e.g. K, M, G.

Fixes #3540.

repos/os/src/server/ram_block/main.cc

@@ -174,7 +174,8 @@ struct Main
 				size = config.attribute_value("size", Number_of_bytes());
 			}
 
-			block_size = config.attribute_value("block_size", block_size);
+			block_size = config.attribute_value("block_size",
+			                                    Number_of_bytes(block_size));
 		}
 
 		Block::Driver *create() override