Support x86_64 hosts

[andr-ec]

All backends currently hardcode qemu-system-aarch64. Any plans to support x86_64 hosts? platform.go already mentions q35 but nothing is wired up yet.

[linear]

PCC-214

[jpmcb]

Good callout - this has been in my backlog: should be able to get to it this weekend! Stay tuned!