qemu -- denial of service vulnerability in MSI-X support

Prasad J Pandit, Red Hat Product Security Team, reports:

Qemu emulator built with the PCI MSI-X support is vulnerable to
null pointer dereference issue. It occurs when the controller
attempts to write to the pending bit array(PBA) memory region.
Because the MSI-X MMIO support did not define the .write method.
A privileges used inside guest could use this flaw to crash the
Qemu process resulting in DoS issue.