virtio_pci_modern: type-safe io accessors

The spec is very clear on this:

4.1.3.1 Driver Requirements: PCI Device Layout

The driver MUST access each field using the “natural” access method,
i.e. 32-bit accesses for 32-bit fields, 16-bit accesses for 16-bit
fields and 8-bit accesses for 8-bit fields.

Add type-safe wrappers to prevent access with incorrect width.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>