selftests: fix check for circular KVM_CAP_VM_MOVE_ENC_CONTEXT_FROM

KVM_CAP_VM_MOVE_ENC_CONTEXT_FROM leaves the source VM in a dead state,
so migrating back to the original source VM fails the ioctl.  Adjust
the test.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>