When reconnecting to a domain started with a host-model CPU which was
started by old libvirt that did not replace host-model with the real CPU
definition, libvirt replaces the host-model CPU with the CPU from
capabilities (because this is what the old libvirt did when it started
the domain). Without this patch libvirt could use features unknown to
QEMU in the CPU definition which replaced the original host-model CPU.
Such domain would keep running just fine, but any attempt to migrate it
will fail and once the domain is saved or snapshotted, restoring it
would fail too.

In other words whenever we want to use the CPU definition from host
capabilities as a guest CPU definition, we have to filter the unknown
features.

https://bugzilla.redhat.com/show_bug.cgi?id=1495171Signed-off-by: NJiri Denemark <jdenemar@redhat.com>
Reviewed-by: NPavel Hrdina <phrdina@redhat.com>