Guest machine freezes when NFS mount goes offline

I have a virtual KVM machine that has 2 CDROM units with ISOs mounted from a NFS mount point. When NFS server goes offline the virtual machine blocks completely instead of throwing read errors for the CDROM device.

Host: Proxmox VE 1.8-11 (Debian GNU/Linux 5.0)
KVM commandline version: QEMU emulator version 0.14.1 (qemu-kvm-devel)
Guest: Windows 7 professional SP 1