Use a devd event to start hv_kvpd instead of doing so in rc.d script.
This is cleaner and eliminates the unneeded startup of KVP daemon on
systems that do not run as a Hyper-V guest.

Submitted by:	hrs
X-MFC-with:	271493, 271688, 271699


git-svn-id: svn+ssh://svn.freebsd.org/base/head@271696 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f