Description of problem:
Following commits are needed by libvirt for the incremental backup feature:

commit ca4bfec41d56a1154da89b105048b3462361d0f0
Author: Peter Krempa <pkrempa>
Date:   Fri Feb 12 18:34:25 2021 +0100

    qemu-iotests: 300: Add test case for modifying persistence of bitmap
    
    Verify that the modification of the bitmap persistence over migration
    which is controlled via BitmapMigrationBitmapAliasTransform works
    properly.
    
    Based on TestCrossAliasMigration
    
    Signed-off-by: Peter Krempa <pkrempa>
    Message-Id: <d9c8e9827e9b6001b2dd1b92e64aab858e6d2a86.1613150869.git.pkrempa>
    Reviewed-by: Eric Blake <eblake>
    [eblake: Adjust test for explicit read_zeroes=False]
    Signed-off-by: Eric Blake <eblake>

commit 6e9f21a2aa8a78bc9a512a836a40c79fe50dd2b4
Author: Peter Krempa <pkrempa>
Date:   Fri Feb 12 18:34:24 2021 +0100

    migration: dirty-bitmap: Allow control of bitmap persistence
    
    Bitmap's source persistence is transported over the migration stream and
    the destination mirrors it. In some cases the destination might want to
    persist bitmaps which are not persistent on the source (e.g. the result
    of merging bitmaps from a number of layers on the source when migrating
    into a squashed image) but currently it would need to create another set
    of persistent bitmaps and merge them.
    
    This patch adds a 'transform' property to the alias map which allows
    overriding the persistence of migrated bitmaps both on the source and
    destination sides.
    
    Signed-off-by: Peter Krempa <pkrempa>
    Message-Id: <b20afb675917b86f6359ac3591166ac6d4233573.1613150869.git.pkrempa>
    Reviewed-by: Eric Blake <eblake>
    [eblake: grammar tweaks, drop dead conditional]
    Signed-off-by: Eric Blake <eblake>

commit 0d1e450c7b3117ee635a00c81d9a92666ebc7ffa
Author: Peter Krempa <pkrempa>
Date:   Fri Feb 12 18:34:23 2021 +0100

    migration: dirty-bitmap: Use struct for alias map inner members
    
    Currently the alias mapping hash stores just strings of the target
    objects internally. In further patches we'll be adding another member
    which will need to be stored in the map so pass a copy of the whole
    BitmapMigrationBitmapAlias QAPI struct into the map.
    
    Signed-off-by: Peter Krempa <pkrempa>
    Message-Id: <fc5f27e1fe16cb75e08a248c2d938de3997b9bfb.1613150869.git.pkrempa>
    Reviewed-by: Eric Blake <eblake>
    [eblake: adjust long lines]
    Signed-off-by: Eric Blake <eblake>

The commits add 'transform' to 'BitmapMigrationBitmapAlias' QAPI struct which allows modification of the persistence state of migrated bitmaps.

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