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

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