msix: make [un]use vectors on reset/load optional

The facility to use/unuse vectors dynamically is helpful
for virtio but little else: everyone just seems to use
vectors in their init function.

Avoid clearing msix vector use info on reset and load.
For virtio, clear it explicitly.
This should fix regressions reported with ivshmem - though
I didn't test this, I verified that virtio keeps
working like it did.

Tested-by: Cam Macdonell <cam@cs.ualberta.ca>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>