qemu: Enabled pause-before-switchover migration capability
    
    QEMU identified a race condition between the device state serialization
    and the end of storage migration. Both QEMU and libvirt needs to be
    updated to fix this.
    
    Our migration work flow is modified so that after starting the migration
    we to wait for QEMU to enter "pre-switchover", "postcopy-active", or
    "completed" state. Once there, we cancel all block jobs as usual. But if
    QEMU is in "pre-switchover", we need to resume the migration afterwards
    and wait again for the real end (either "postcopy-active" or
    "completed" state).
    
    Old QEMU will just enter either "postcopy-active" or "completed"
    directly, which is still correctly handled even by new libvirt. The
    "pre-switchover" state will only be entered if QEMU supports it and the
    pause-before-switchover capability was enabled. Thus all combinations of
    libvirt and QEMU will work, but only new QEMU with new libvirt will
    avoid the race condition.
    
    Signed-off-by: Jiri Denemark <jdenemar@redhat.com>