qemu: Refactor qemuMigrationWaitForCompletion
    
    Checking status of all part of migration and aborting it when something
    failed is a complex thing which makes the waiting loop hard to read.
    This patch moves all the checks into a separate function similarly to
    what was done for drive mirror loops.
    
    Signed-off-by: Jiri Denemark <jdenemar@redhat.com>