Libvirt relies on being able to kill the destination domain and resume
the source one during migration until we called "cont" on the
destination. Unfortunately, QEMU automatically activates block devices
at the end of migration even when it's called with -S. This wasn't a big
issue in the past since the guest is not running and thus no data are
written to the block devices. However, when QEMU introduced its internal
block device locks, we can no longer resume the source domain once the
destination domain already activated the block devices (and thus
acquired all locks) unless the destination domain is killed first.

Since it's impossible to synchronize the destination and the source
libvirt daemons after a failed migration, QEMU introduced a new
migration capability called "late-block-activate" which ensures QEMU
won't activate block devices until it gets "cont". The only thing we
need to do is to enable this capability whenever QEMU supports it.

https://bugzilla.redhat.com/show_bug.cgi?id=1568407

QEMU commit implementing the capability: v2.12.0-952-g0f073f44df
Signed-off-by: NJiri Denemark <jdenemar@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>