KVM: SVM: Restore tracing of nested vmcb address

A recent change broke tracing of the nested vmcb address. It
was reported as 0 all the time. This patch fixes it.

Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>
Signed-off-by: Avi Kivity <avi@redhat.com>