qemu builds xen-specific package now with xen support, with qemu binary being /usr/libexec/xen-qemu-system-i386. Stop recommending generic qemu-system-x86 with a lot of dependencies, move to much leanier qemu-system-xen build.
Only recommend on x86 (on amd64), since it is x86-specicic.
While at it, fix qemu arguments in the startup script, it is -monitor none, not /dev/null.