kvmtool: vhost-net: fix ioeventfd registration

On registering the ioeventfds for the virtio-pci device we cover both
the I/O ports and the MMIO BAR.
But as the current code advertises both as PIO, the host kernel gets
the wrong bus number for the MMIO region.
Fix the issue by marking only the actual PIO area as PIO.
This fixes vhost-net on x86.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>