Move the last_bytes util method to libvirt

The only user of this util method is libvirt, so it should live there.
This is a precursor to re-writing its single caller to use privsep to
read bytes instead of chown'ing files.

Change-Id: Ice5cc70eeef3fbeee9a1a0ec0daa801c6f74480c

nova/utils.py

@@ -20,7 +20,6 @@
 import contextlib
 import copy
 import datetime
-import errno
 import functools
 import hashlib
 import inspect
@@ -837,30 +836,6 @@ def mkfs(fs, path, label=None, run_as_root=False):
     execute(*args, run_as_root=run_as_root)
 
 
-def last_bytes(file_like_object, num):
-    """Return num bytes from the end of the file, and remaining byte count.
-
-    :param file_like_object: The file to read
-    :param num: The number of bytes to return
-
-    :returns: (data, remaining)
-    """
-
-    try:
-        file_like_object.seek(-num, os.SEEK_END)
-    except IOError as e:
-        # seek() fails with EINVAL when trying to go before the start of the
-        # file. It means that num is larger than the file size, so just
-        # go to the start.
-        if e.errno == errno.EINVAL:
-            file_like_object.seek(0, os.SEEK_SET)
-        else:
-            raise
-
-    remaining = file_like_object.tell()
-    return (file_like_object.read(), remaining)
-
-
 def metadata_to_dict(metadata, include_deleted=False):
     result = {}
     for item in metadata: