virtio/pci: Ignore MMIO and I/O accesses when they are disabled

A device's response to memory or I/O accesses is disabled when Memory
Space, respectively I/O Space, is set to 0 in the Command register.
According to the PCI Local Bus Specification Revision 3.0, those two
bits reset to 0.

Let's respect the specifiction, so set Command and I/O Space to 0 on
reset, and ignore accesses to a device's respective regions when they
are disabled.