Documentation: KVM: add virtual CPU errata documentation

Add a file to document all the different ways in which the virtual CPU
emulation is imperfect.  Include an example to show how to document
such errata.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Jim Mattson <jmattson@google.com>
Reviewed-by: Oliver Upton <oupton@google.com>
Message-Id: <20220322110712.222449-3-pbonzini@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>