qemu: allow specifying the machine type

[IsaacJT]

This allows QEMU machine types to be used other than the default "pc", such as "q35". Valid alternative machine types can be identified with the qemu-system-x86_64 -M help command.

This is useful for testing images for embedded systems which may not support the "pc" QEMU machine type, or require features from "q35" or other types.