qemu: Add virConnectPtr back to some migration methods
    
    This partially reverts 82592551cb8c4112cfa2264d50b8dce5349533d5.
    
    When migrating a domain, qemuMigrationDstPrepareAny() is called
    which eventually calls qemuProcessLaunch(conn = NULL, flags =
    VIR_QEMU_PROCESS_START_AUTODESTROY); But the very first thing
    that qemuProcessLaunch does is check if AUTODESTROY flag is set
    and @conn is not NULL. Well, it is.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
    Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>