igb: Check if subordinate VFs are assigned to virtual machines

Kvm and the Xen pci-back driver will set a flag in the virtual function
pci device dev_flags when the VF is assigned to a guest VM.  Before
destroying subordinate VFs check to see if the flag is set and if so
skip the call to pci_disable_sriov() to avoid system crashes.

Copy the maintainer for the Xen pci-back driver.  Also CC'ing
maintainers of all drivers found to call pci_disable_sriov().

V2 - Fix  uninitialized variable warning

Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Christian Benvenuti <benve@cisco.com>
Cc: Sathya Perla <sathya.perla@emulex.com>
Cc: Dimitris Michailidis <dm@chelsio.com>
Cc: Jon Mason <jdmason@kudzu.us>
Cc: James Smart <james.smart@emulex.com>
Signed-off-by: Greg Rose <gregory.v.rose@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>