The qemu-system-i386 emulator is always used, regardless of the architecture being installed. So, while confusing, virt-manager's emulator specification is actually correct.
Further to my previous message, it appears that the nominal architecture
makes no difference to qemu, as it emulates no CPU instructions:
https://wiki.xen.org/wiki/QEMU_Upstream#Why_is_qemu-system-i386_used_even_on_x86_64_and_even_non-x86.3F
