Add missing hvdos public domain attribution:

hvf.c and vmx.h contain code from hvdos.c that is released as public domain:

from hvdos github: https://github.com/mist64/hvdos



"License

See LICENSE.txt (2-clause-BSD).

In order to simplify use of this code as a template, you can consider any parts from "hvdos.c" and "interface.h" as being in the public domain."

Signed-off-by: Izik Eidus <izik@veertu.com>
Message-Id: <20180123123639.35255-2-izik@veertu.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>