virt: detect Alibaba Cloud ECS as KVM

Alibaba Cloud ECS instances expose "Alibaba Cloud ECS" in DMI
product_name/bios_vendor and "Alibaba Cloud" in sys_vendor. None of
these match dmi_vendor_table[], so systemd-detect-virt --vm reports
"none" on these machines, even though they run on a KVM-based
hypervisor. This is especially visible on ARM64 instances, where the
CPUID fallback does not exist (x86 only) and /proc/device-tree is
not populated.

Add "Alibaba Cloud ECS" to dmi_vendor_table[], mapping to
VIRTUALIZATION_KVM. The narrower prefix avoids matching the bare
"Alibaba Cloud" sys_vendor value, reducing the false-positive
surface for Alibaba's bare-metal (X-Dragon) instances compared to
the original broader prefix. bios_vendor also carries the same
string on ECS VMs though, so the protection is not complete —
fully resolving the VM-vs-bare-metal ambiguity would need the
SMBIOS BIOS-Characteristics bit check that the Amazon EC2 entry
uses.

The existing "back to DMI" fallback path in detect_vm() reports
"kvm" on both x86 and ARM64.

src/basic/virt.c

@@ -180,6 +180,7 @@ static Virtualization detect_vm_dmi_vendor(void) {
                 { "KVM",                   VIRTUALIZATION_KVM       },
                 { "OpenStack",             VIRTUALIZATION_KVM       }, /* Detect OpenStack instance as KVM in non x86 architecture */
                 { "KubeVirt",              VIRTUALIZATION_KVM       }, /* Detect KubeVirt instance as KVM in non x86 architecture */
+                { "Alibaba Cloud ECS",     VIRTUALIZATION_KVM       }, /* narrower than bare "Alibaba Cloud" sys_vendor */
                 { "Amazon EC2",            VIRTUALIZATION_AMAZON    },
                 { "QEMU",                  VIRTUALIZATION_QEMU      },
                 { "VMware",                VIRTUALIZATION_VMWARE    }, /* https://kb.vmware.com/s/article/1009458 */