Sure enough, someone on the qemu-ppc mailing list was kind enough to point out the correct invocation to manually specify the pcie bus for a 9pnet_virtio device.
For anyone who may come across this in the future, instead of the abbreviated form:
-virtfs local,path=/nix/store,security_model=none,mount_tag=store
we need to use the long form to specify the pcie bus:
-fsdev local,id=fsdev1,path=/nix/store,security_model=none
-device virtio-9p-pci,fsdev=fsdev1,mount_tag=store,bus=pcie.1
My VM is now able to fully boot 