`x86_64-xen-pv`: add instructions for running oxerun

[n0toose]

Clarified here for aarch64:

xen/oxerun/README.md

Lines 37 to 56 in 46cbbcd

	## Running the unikernel on QEMU:
	```shell
	$ qemu-system-aarch64 --version
	QEMU emulator version 6.0.94 (v6.1.0-rc4-2-gecf5e5ec1f44)
	Copyright (c) 2003-2021 Fabrice Bellard and the QEMU Project developers
	$
	$ qemu-system-aarch64 \
	-machine virt,virtualization=on \
	-cpu cortex-a57 -serial mon:stdio \
	-device virtio-net-pci,netdev=net0 \
	-netdev user,id=net0,hostfwd=tcp::8022-:22 \
	-device virtio-scsi-pci \
	-drive file=debian-buster-arm64,index=0,id=hd0,if=none,format=raw \
	-device scsi-hd,drive=hd0 \
	-display none \
	-m 8192 -smp 4\
	-kernel $(PATH_TO_XEN)/xen/xen \
	-device guest-loader,addr=0x80000000,kernel=$(PATH_TO_XEN_SYS)/xen-sys/target/aarch64-xen-hvm/release/oxerun.bin
	```

This is otherwise not done for x86_64-xen-pv - which makes some sense, given that it's not an HVM target. Still, some guidance could be worth documenting.