Until now host-model CPU mode tried to enable all CPU features supported
by the host CPU even if QEMU/KVM did not support them. This caused a
number of issues and made host-model quite unreliable. Asking QEMU for
the CPU it can provide and the current host makes host-model much more
robust.

This commit fixes the following bugs:

    https://bugzilla.redhat.com/show_bug.cgi?id=1018251
    https://bugzilla.redhat.com/show_bug.cgi?id=1371617
    https://bugzilla.redhat.com/show_bug.cgi?id=1372581
    https://bugzilla.redhat.com/show_bug.cgi?id=1404627
    https://bugzilla.redhat.com/show_bug.cgi?id=870071

In addition to that, the following bug should be mostly limited to cases
when an unsupported feature is explicitly requested:

    https://bugzilla.redhat.com/show_bug.cgi?id=1335534Signed-off-by: NJiri Denemark <jdenemar@redhat.com>