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>