KVM: don't boot from cdrom with no cdrom
KVM: don't boot from cdrom with no cdrom

Reviewed-by: iustinp


mbakke/ganeti,yiannist/ganeti,grnet/snf-ganeti,bitemyapp/ganeti,dimara/ganeti,andir/ganeti,mbakke/ganeti,leshchevds/ganeti,ganeti/ganeti,ganeti/ganeti,leshchevds/ganeti,andir/ganeti,leshchevds/ganeti,onponomarev/ganeti,bitemyapp/ganeti,ganeti-github-testing/ganeti-test-1,apyrgio/ganeti,apyrgio/ganeti,grnet/snf-ganeti,onponomarev/ganeti,dimara/ganeti,apyrgio/snf-ganeti,yiannist/ganeti,mbakke/ganeti,yiannist/ganeti,andir/ganeti,ganeti-github-testing/ganeti-test-1,ganeti/ganeti,apyrgio/snf-ganeti