If userspace creates and destroys multiple VMs within the same process
we leak 20k of memory in the userspace process context per VM.  This
patch frees the memory in kvm_arch_destroy_vm.  If the process exits
without closing the VM file descriptor or the file descriptor has been
shared with another process then we don't free the memory.

It's still possible for a user space process to leak memory if the last
process to close the fd for the VM is not the process that created it.
However, this is an unexpected case that's only caused by a user space
process that's misbehaving.

Signed-off-by: Andrew Honig <ahonig@google.com>
Signed-off-by: Gleb Natapov <gleb@redhat.com>