bhyve vtblk: Inform guests of disk resize events.

Register a resize callback with the blockif interface. When the
callback fires, update the size of the disk and notify the guest via a
configuration change interrupt.

Reviewed by: grehan, markj
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D30506

(cherry picked from commit 2349cda44fea17c089b026e743ec8f03199019eb)