backport of the upstream kernel commit da8d1c8ba4dcb16d60be54b233deca9a7cac98dc

commit da8d1c8ba4dcb16d60be54b233deca9a7cac98dc
Author: Neil Horman <nhorman>
Date:   Thu Oct 6 14:08:18 2011 -0400

    PCI/sysfs: add per pci device msi[x] irq listing (v5)
    
    This patch adds a per-pci-device subdirectory in sysfs called:
    /sys/bus/pci/devices/<device>/msi_irqs
    
    This sub-directory exports the set of msi vectors allocated by a given
    pci device, by creating a numbered sub-directory for each vector beneath
    msi_irqs.  For each vector various attributes can be exported.
    Currently the only attribute is called mode, which tracks the
    operational mode of that vector (msi vs. msix)
    
    Acked-by: Greg Kroah-Hartman <gregkh>
    Signed-off-by: Jesse Barnes <jbarnes>

this is required bits for the feature request from bug 710576, in priority list.