Save and restore SRIOV-related config registers.

Save the value of the IOV control and page size registers and restore
them (along with the VF count) in pci_cfg_save/pci_cfg_restore. This
ensures ARI remains enabled if a PF driver resets itself during the
PCI_IOV_INIT callback. This might also properly restore SRIOV state
across suspend/resume.

Reviewed by: rstone, vangyzen
Differential Revision: https://reviews.freebsd.org/D6192