Handbook/Virtualization: Expand bhyve documentation

Notable changes:

• describe the experimental snapshot feature
• add notice about persisting and changing network configuration and the potential for locking oneself out
• describe jailing bhyve
• improve wording throughout the page

Reviewed by: emaste, jrm, Pau Amma <pauamma@gundo.com>
Differential Revision: https://reviews.freebsd.org/D44078