i386: define the AMD 'virt-ssbd' CPUID feature bit (CVE-2018-3639)

AMD Zen expose the Intel equivalant to Speculative Store Bypass Disable
via the 0x80000008_EBX[25] CPUID feature bit.

This needs to be exposed to guest OS to allow them to protect
against CVE-2018-3639.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20180521215424.13520-3-berrange@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
(cherry picked from commit 403503b162ffc33fb64cfefdf7b880acf41772cd)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>

target/i386/cpu.c

@@ -490,7 +490,7 @@ static FeatureWordInfo feature_word_info[FEATURE_WORDS] = {
             "ibpb", NULL, NULL, NULL,
             NULL, NULL, NULL, NULL,
             NULL, NULL, NULL, NULL,
-            NULL, NULL, NULL, NULL,
+            NULL, "virt-ssbd", NULL, NULL,
             NULL, NULL, NULL, NULL,
         },
         .cpuid_eax = 0x80000008,