virtio/pci.c registers a notification ioeventfd on both PIO and MMIO
buses. But architectures other than x86 cannot differentiate MMIO from
PIO traps, and the kernel always calls kvm_io_bus_read/write with
KVM_MMIO_BUS as argument.

As a result kvmtool's ioeventfd isn't used with virtio PCI, because the
kernel can't find it and all accesses to the doorbell return to
userspace. To fix it, don't set the PIO flag if the architecture doesn't
support it.

Fixes: a508ea95

 ("virtio/pci: Use port I/O for configuration registers by default")
Signed-off-by: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>