KVM: Use kmem_cache_free for kmem_cache_zalloc'ed objects

We use kfree in svm.c and vmx.c, and this works, but it could break at
any time.  kfree() is supposed to match up with kmalloc().

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Avi Kivity <avi@qumranet.com>