bhyveload(8): Implement loader_callbacks::diskwrite

The method was optional prior to r365938, which made it mandatory but did add
any test that an implementation provides the method nor implement it for
bhyveload. The code path might not be hit unless the user's loader was
configured to write to a file on disk, such as with nextboot(8).

Reviewed by: grehan, tsoome
Approved by: bhyve
X-MFC-With: r365938
Differential Revision: https://reviews.freebsd.org/D26710