kvm: x86: vmx: fix vpid leak

VPID for the nested vcpu is allocated at vmx_create_vcpu whenever nested
vmx is turned on with the module parameter.

However, it's only freed if the L1 guest has executed VMXON which is not
a given.

As a result, on a system with nested==on every creation+deletion of an
L1 vcpu without running an L2 guest results in leaking one vpid.  Since
the total number of vpids is limited to 64k, they can eventually get
exhausted, preventing L2 from starting.

Delay allocation of the L2 vpid until VMXON emulation, thus matching its
freeing.

Fixes: 5c614b35


Cc: stable@vger.kernel.org
Signed-off-by: Roman Kagan <rkagan@virtuozzo.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>