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

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

BugLink: http://bugs.launchpad.net/bugs/1734130
$ 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>
(cherry picked from commit 0866c31bf32fbd05dd25b6ed4976fa9134773227 linux-next)
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
Acked-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>