target/i386: add MDS-NO feature

target/i386: add MDS-NO feature

Microarchitectural Data Sampling is a hardware vulnerability which allows
unprivileged speculative access to data which is available in various CPU
internal buffers.

Some Intel processors use the ARCH_CAP_MDS_NO bit in the
IA32_ARCH_CAPABILITIES
MSR to report that they are not vulnerable, make it available to guests.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20190516185320.28340-1-pbonzini@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>