virtio-pci: build for uninitialized return value in vq_vector_unmask

Fixes the following:

/home/mdroth/w/qemu2.git/hw/virtio-pci.c: In function
‘kvm_virtio_pci_vector_unmask’:
/home/mdroth/w/qemu2.git/hw/virtio-pci.c:673:12: error: ‘ret’ may be
used uninitialized in this function [-Werror=uninitialized]
cc1: all warnings being treated as errors
make: *** [hw/virtio-pci.o] Error 1
make: *** Waiting for unfinished jobs....

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
