virtio/pci: Make memory and IO BARs independent

Currently, callbacks for memory BAR 1 call the IO port emulation.  This
means that the memory BAR needs I/O Space to be enabled whenever Memory
Space is enabled.

Refactor the code so the two type of  BARs are independent. Also, unify
ioport/mmio callback arguments so that they all receive a virtio_device.

Signed-off-by: Julien Thierry <julien.thierry@arm.com>