Also added optional affinity Python package for CPU pinning under KVM
to the INSTALL doc.

Signed-off-by: Tsachy Shacham <tsachy@google.com>
Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>