random(4): Fix RANDOM_LOADABLE build

I introduced an obvious compiler error in r346282, so this change fixes
that.

Unfortunately, RANDOM_LOADABLE isn't covered by our existing tinderbox, and
it seems like there were existing latent linking problems. I believe these
were introduced on accident in r338324 during reduction of the boolean
expression(s) adjacent to randomdev.c and hash.c. It seems the
RANDOM_LOADABLE build breakage has gone unnoticed for nine months.

This change correctly annotates randomdev.c and hash.c with !random_loadable
to match the pre-r338324 logic; and additionally updates the HWRNG drivers
in MD 'files.*', which depend on random_device symbols, with
!random_loadable (it is invalid for the kernel to depend on symbols from a
module).

(The expression for both randomdev.c and hash.c was the same, prior to
r338324: "optional random random_yarrow | random !random_yarrow
!random_loadable". I.e., "random && (yarrow || !loadable)." When Yarrow
was removed ("yarrow := False"), the expression was incorrectly reduced to
"optional random" when it should have retained "random && !loadable".)

Additionally, I discovered that virtio_random was missing a MODULE_DEPEND on
random_device, which breaks kld load/link of the driver on RANDOM_LOADABLE
kernels. Address that issue as well.

(cherry picked from commit 5ca5dfe938c1e4c741dc4c7bd83c738714456978)