fix: make aarch64_with_sve_and_pac template work with >=2 vcpus

Setting the least significant 4 bits to zero overwrites some
configuration that Firecracker sets on secondary CPUs that is needed for
them to be able to be booted. With these 4 bits set to 0, the CPUs will
never show up as online inside the guest, and KVM will just infinitely
spin inside KVM_RUN for them. Fix by using 'x' in the template, which
preserves the defaults set by Firecracker.

Signed-off-by: Patrick Roy <[email protected]>

tests/data/static_cpu_templates/aarch64_with_sve_and_pac.json

@@ -1,4 +1,4 @@
 {
   "kvm_capabilities": ["170", "171", "172"],
-  "vcpu_features": [{ "index": 0, "bitmap": "0b1110000" }]
+  "vcpu_features": [{ "index": 0, "bitmap": "0b111xxxx" }]
 }