Q: skip buildworld, just buildkernel and stop at install kernel (no result from image/_.disk)
A: 
cp's comment: sometimes cannot just skip build world, go directly build kernel

check logging
# tail /usr/embedded/qemu-i386/_.iw# tail /usr/embedded/qemu-i386/_.iw
--

Setting variable: __MAKE_CONF="/usr/embedded/obj/make.conf.install"
+ cd /usr/src
+ make installworld 'DESTDIR=/usr/embedded/qemu-i386/_.w' 'DB_FROM_SRC=yes'
make[1]: "/usr/src/Makefile.inc1" line 97: A build is required first.  You may have the wrong MAKEOBJDIRPREFIX set.
*** Error code 1