Add qemu as optional dependency for systemd-vmspawn

Description:

systemd-vmspawn introduced with systemd 255, requires QEMU to be installed.

For this reason I think it make sense to add QEMU as a optional dependency. qemu-base should cut it. If starting QEMU "in graphical mode" is desired (--qemu-gui), qemu-hw-display-virtio-gpu and qemu-hw-display-virtio-vga should also be installed.

systemd-vmspawn is broken for unrelated reasons on arch at the moment (upstream issue), which should be fixed in the next systemd release.

Additional info:

• package version(s): 255.4-2

Steps to reproduce:

1. Run: systemd-vmspawn --image=image.raw
2. It fails with: Failed to find QEMU binary: No such file or directory