block/iscsi: validate block size returned from target

It has been reported that at least tgtd returns a block size of 0
for LUN 0. To avoid running into divide by zero later on and protect
against other problematic block sizes validate the block size right
at connection time.

Cc: qemu-stable@nongnu.org
Reported-by: Andrey Korolyov <andrey@xdel.ru>
Signed-off-by: Peter Lieven <pl@kamp.de>
Message-Id: <1439552016-8557-1-git-send-email-pl@kamp.de>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
