KVM: set no_llseek in stat_fops_per_vm

In vm_stat_get_per_vm_fops and vcpu_stat_get_per_vm_fops, since we
use nonseekable_open() to open, we should use no_llseek() to seek,
not generic_file_llseek().

Signed-off-by: Geliang Tang <geliangtang@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>