Detect XML in *_check tools (#86)

* [*_check] Detect XML in cache_check and era_check

This is based on previous commit b10d8d4440.

* [*_check] Fix typo in check_superblock

persistent-data/file_utils.h

@@ -10,6 +10,7 @@
 
 // FIXME: move to a different unit
 namespace persistent_data {
+	bool check_for_xml(block_manager<>::ptr bm);
 	persistent_data::block_address get_nr_blocks(std::string const &path, sector_t block_size = MD_BLOCK_SIZE);
 	block_address get_nr_metadata_blocks(std::string const &path);