KVM: only pass boot flag once

This addresses issue 230: passing two methods of booting to KVM can,
depending on the KVM version, confuse it.

Note that commit 4304964a introduced a partial fix for this (but only
for disks, and keyed on KVM versions). However, it didn't fix cdrom
booting, which still fails with the same error, so let's fix it more
generically; we still leave the per-disk check since that is about
-boot c versus -drive …,boot=on rather than two boot methods.

Patch is based on the one submitted by Vladimir Mencl, many thanks!

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>