Move nested_get_vpid02() to vmx/nested.h so that a future patch can
reference it from vmx.c to implement context-specific TLB flushing.

No functional change intended.
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Message-Id: <20200320212833.3507-20-sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>