KVM: x86: nSVM: implement nested LBR virtualization

This was tested with kvm-unit-test that was developed
for this purpose.

Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
Message-Id: <20220322174050.241850-3-mlevitsk@redhat.com>
[Copy all of DEBUGCTL except for reserved bits. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>