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.cc

@@ -7,6 +7,7 @@
 #include <sys/types.h>
 #include <sys/stat.h>
 #include <fcntl.h>
+#include <string.h>
 #include <sstream>
 #include <unistd.h>
 
@@ -17,6 +18,14 @@ using namespace std;
 
 //----------------------------------------------------------------
 
+bool
+persistent_data::check_for_xml(block_manager<>::ptr bm) {
+	block_manager<>::read_ref b = bm->read_lock(0);
+	const char *data = reinterpret_cast<const char *>(b.data());
+	return (!strncmp(data, "<superblock", 11) || !strncmp(data, "<?xml", 5)
+	                       || !strncmp(data, "<!DOCTYPE", 9));
+}
+
 persistent_data::block_address
 persistent_data::get_nr_blocks(std::string const &path, sector_t block_size)
 {