The MITRE CVE dictionary describes this issue as:
The pciej_write function in hw/acpi_piix4.c in the PIIX4 Power Management emulation in qemu-kvm does not check if a device is hotpluggable before unplugging the PCI-ISA bridge, which allows privileged guest users to cause a denial of service (guest crash) and possibly execute arbitrary code by sending a crafted value to the 0xae08 (PCI_EJ_BASE) I/O port, which leads to a use-after-free related to "active qemu timers."
Find out more about CVE-2011-1751 from the
MITRE CVE dictionary dictionary and
This issue only affects Red Hat Enterprise Linux 6. The version of the qemu/kvm as shipped with Red Hat Enterprise Linux 5 is not affected.