Support Hyper-V on i386:

 - Add 'hyperv' module into build;
 - Allow building Hyper-V support as part of the kernel;
 - Hook Hyper-V build into NOTES.

This is intended for MFC if re@ permits.

MFC after:	3 days
Ensure WITH_DVD is not empty to prevent 'WITH_DVD= ' from evaluating
to true.

Submitted by:	hrs
MFC after:	3 days
X-MFC-to-10:	immediate
Sponsored by:	The FreeBSD Foundation
Enable Hyper-V support in i386 GENERIC.

MFC after:	2 weeks