target/i386: Disable MPX support on named CPU models

target/i386: Disable MPX support on named CPU models

MPX support is being phased out by Intel; GCC has dropped it, Linux
is also going to do that.  Even though KVM will have special code
to support MPX after the kernel proper stops enabling it in XCR0,
we probably also want to deprecate that in a few years.  As a start,
do not enable it by default for any named CPU model starting with
the 4.0 machine types; this include Skylake, Icelake and Cascadelake.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20181220121100.21554-1-pbonzini@redhat.com>
Reviewed-by:   Wainer dos Santos Moschetta <wainersm@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>