bhyve: Use fspacectl(2) for BOP_DELETE on regular file images

bhyve can also make use of fspacectl(2) to implement BOP_DELETE with
hole-punching. Since it is not desirable to do zero-filling for large
DEALLOCATE/UNMAP range, candelete is not set if pathconf(2) indicates
that the underlying file system does not support native
VOP_DEALLOCATE(9).

Sponsored by: The FreeBSD Foundation
Reviewed by: grehan
Differential Revision: https://reviews.freebsd.org/D28880