grub: xen installer, use kvm flavor #1806

Job	Run time
packages (arm64-secure, arm64)	11m 36s
packages (ubuntu-20.04, amd64)	14m 17s
packages (ubuntu-latest, riscv64)	50m 55s
verification (arm64, kvm)	11m 20s
manifest	3m 55s
verification (arm64, xen)	10m 33s
eve (arm64, kvm)	28m 26s
verification (amd64, kvm)	10m 58s
eve (arm64, xen)	32m 49s
verification (amd64, xen)	10m 17s
eve (amd64, kvm)	32m 25s
eve (amd64, xen)	30m 8s
eve (riscv64, mini)	4m 25s
trigger_assets	0s
	4h 12m 4s

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