tools/kvm_stat: Don't use deprecated file()

$ python3 tools/kvm/kvm_stat/kvm_stat
Traceback (most recent call last):
  File "tools/kvm/kvm_stat/kvm_stat", line 1668, in <module>
    main()
  File "tools/kvm/kvm_stat/kvm_stat", line 1639, in main
    assign_globals()
  File "tools/kvm/kvm_stat/kvm_stat", line 1618, in assign_globals
    for line in file('/proc/mounts'):
NameError: name 'file' is not defined

open() is the python3 way, and works on python2.6+
Signed-off-by: Cole Robinson <crobinso@redhat.com>
Reviewed-and-tested-by: Stefan Raspl <stefan.raspl@linux.vnet.ibm.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>

tools/kvm/kvm_stat/kvm_stat

@@ -1615,7 +1615,7 @@ def assign_globals():
     global PATH_DEBUGFS_TRACING
 
     debugfs = ''
-    for line in file('/proc/mounts'):
+    for line in open('/proc/mounts'):
         if line.split(' ')[0] == 'debugfs':
             debugfs = line.split(' ')[1]
             break