x86: check if desc->action is NULL when unbinding guest pirq

x86: check if desc->action is NULL when unbinding guest pirq

Before igb PF driver is unloaded, dom0 doesn't unload igbvf driver
automatically. When igb drver is unloaded, it invokes the
PHYSDEVOP_manage_pci_remove hypercall to remove the VFs and xen frees
the msi irqs by pci_cleanup_msi() -> ... -> dynamic_irq_cleanup() and
sets the desc->action to NULL.  igbvf driver knows the VF is
disappearing via a hook ndo_stop() in dev_close() and tries to unbind
the pirq and xen would crash as the desc->action is NULL now.

Signed-off-by: Dexuan Cui <dexuan.cui@intel.com>