qemu: Don't use legacy USB for aarch64 mach-virt guests

qemu: Don't use legacy USB for aarch64 mach-virt guests

The '-usb' option doesn't have any effect for aarch64 mach-virt
guests, so the fact that it's currently enabled by default is not
really causing any issue.

However, that might change in the future (although unlikely), and
having it as part of the QEMU command line can cause confusion to
someone looking through the process list.

Avoid it completely, like it's already happening for q35.