i386: Add Cascadelake-Server-v2 CPU model

Add new version of Cascadelake-Server CPU model, setting
stepping=5 and enabling the IA32_ARCH_CAPABILITIES MSR
with some flags.

The new feature will introduce a new host software requirement,
breaking our CPU model runnability promises. This means we can't
enable the new CPU model version by default in QEMU 4.1, because
management software isn't ready yet to resolve CPU model aliases.
This is why "pc-*-4.1" will keep returning Cascadelake-Server-v1
if "-cpu Cascadelake-Server" is specified.

Includes a test case to ensure the right combinations of
machine-type + CPU model + command-line feature flags will work
as expected.

Backports commit fd63c6d1a5f77d689ee06f6561677c012a988223 from qemu

qemu/target/i386/cpu.c

@@ -2451,6 +2451,20 @@ static X86CPUDefinition builtin_x86_defs[] = {
             CPUID_6_EAX_ARAT,
         .xlevel = 0x80000008,
         .model_id = "Intel Xeon Processor (Cascadelake)",
+        .versions = (X86CPUVersionDefinition[]) {
+            { .version = 1 },
+            { .version = 2,
+              .props = (PropValue[]) {
+                  { "arch-capabilities", "on" },
+                  { "rdctl-no", "on" },
+                  { "ibrs-all", "on" },
+                  { "skip-l1dfl-vmentry", "on" },
+                  { "mds-no", "on" },
+                  { /* end of list */ }
+              },
+            },
+            { /* end of list */ }
+        }
     },
     {
         .name = "Icelake-Client",