pmb.qemu.run: workaround for new mkinitfs requirements
It's been 3 months since we switched to new mkinitfs and we are still fixing consequences.
linux-lts and linux-virt used by qemu-amd64 device package install vmlinuz-lts (vmlinuz-virt). And qemu run cmdline passed -kernel vmlinuz which makes it impossible to run qemu without this change.
With this change proper kernel arg is passed to qemu.