[virtio] Fix implementation of vpm_ioread32()

The current implementation of vpm_ioread32() erroneously reads only 16
bits of data, which fails when used with the (stricter) virtio device
emulation in VirtualBox.

Fix by using the correct readl()/inl() I/O wrappers.

Reworded-by: Michael Brown <mcb30@ipxe.org>
Signed-off-by: Michael Brown <mcb30@ipxe.org>