docs: amd-memory-encryption.rst get rid of warnings

Get rid of those warnings:

    Documentation/virtual/kvm/amd-memory-encryption.rst:244: WARNING: Citation [white-paper] is not referenced.
    Documentation/virtual/kvm/amd-memory-encryption.rst:246: WARNING: Citation [amd-apm] is not referenced.
    Documentation/virtual/kvm/amd-memory-encryption.rst:247: WARNING: Citation [kvm-forum] is not referenced.

For references that aren't mentioned at the text by adding an
explicit reference to them.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>