scsi-disk: fix VERIFY emulation

VERIFY emulation was completely botched (and remained botched through
all the refactorings).  The command must be emulated both in check-medium
mode (BYTCHK=00, which we implement by doing nothing) and in check-bytes
mode (which we do not implement yet).  Unlike WRITE AND VERIFY (which we
treat simply as WRITE with FUA bit set), VERIFY cannot be handled like
READ.  In fact the device is _receiving_ data for VERIFY, not _sending_
it like READ.

Cc: qemu-stable@nongnu.org
Tested-by: Hervé Poussineau <hpoussin@reactos.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>