Changed passing argument as "0 to NULL" which resolves below sparse warning

arch/x86/kvm/x86.c:3096:61: warning: Using plain integer as NULL pointer
Signed-off-by: NHariprasad Kelam <hariprasad.kelam@gmail.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>