Spin-off syscall-related file operations (#78)

* [file_utils] spin-off syscall-related file operations

1. Eliminate the potential circular dependency between
   persistent-data/block.h and persistent-data/file_utils.h,
   if the former one wants to include the latter.
2. Avoid namespace pollution by removing the "using namespace std"
   declaration in block.tcc.
3. Correct the header hierarchy: base/xml_utils.h now no longer
   depends on the higher-level persistent-data/file_utils.h

* [file_utils] support block files in get_file_length()

thin-provisioning/thin_trim.cc

@@ -1,8 +1,10 @@
 #include <iostream>
 #include <getopt.h>
 #include <sys/ioctl.h>
+#include <sys/stat.h>
 #include <linux/fs.h>
 #include <libgen.h>
+#include <fcntl.h>
 
 #undef BLOCK_SIZE