qemu-{amd64,aarch64}: add option to use Alpine's linux-lts/linux-virt

Alpine's standard linux-lts and linux-virt kernels are properly maintained with stable kernel updates and work just fine for QEMU.

Eventually I would like to remove linux-postmarketos-{stable,mainline} because we don't maintain them very well. Alpine updates these two kernels regularly. For devices that do not need any WIP changes in the kernel we can just add the needed config options upstream and then use the common kernel. (No need to duplicate the maintenance work...)

This is really a separate issue though (I will open an issue for this at some point). For now, this MR just adds additional options to use -lts/virt.

I tested all kernels briefly and they all boot to console on the none UI at least. I think @afontain might have done some more testing?
qemu-vexpress (armv7) seems entirely broken at the moment. I will try to fix it at some point (likely by attempting to make it more similar to qemu-aarch64.