Revert "virtio_pci: don't duplicate the msix_enable flag in struct pci_dev"

This reverts commit 53a020c6.

The cleanup seems to be one of the changes that broke
hybernation for some users. We are still not sure why
but revert helps.

Tested-by: Mike Galbraith <efault@gmx.de>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>