cpu: define the 'ssbd' CPUID feature bit (CVE-2018-3639)

New microcode introduces the "Speculative Store Bypass Disable"
CPUID feature bit. This needs to be exposed to guest OS to allow
them to protect against CVE-2018-3639.
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>
Reviewed-by: NJiri Denemark <jdenemar@redhat.com>

src/cpu/cpu_map.xml

@@ -298,6 +298,9 @@
     <feature name='spec-ctrl'>
       <cpuid eax_in='0x07' ecx_in='0x00' edx='0x04000000'/>
     </feature>
+    <feature name='ssbd'>
+      <cpuid eax_in='0x07' ecx_in='0x00' edx='0x80000000'/>
+    </feature>
 
     <!-- Processor Extended State Enumeration sub leaf 1 -->
     <feature name='xsaveopt'>