KVM: nVMX: Fix bug preventing more than two levels of nesting

The nested VMX feature is supposed to fully emulate VMX for the guest. This
(theoretically) not only allows it to run its own guests, but also also
to further emulate VMX for its own guests, and allow arbitrarily deep nesting.

This patch fixes a bug (discovered by Kevin Tian) in handling a VMLAUNCH
by L2, which prevented deeper nesting.

Deeper nesting now works (I only actually tested L3), but is currently
*absurdly* slow, to the point of being unusable.

Signed-off-by: Nadav Har'El <nyh@il.ibm.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>