qemuMigrationDstPrepareAnyBlockDirtyBitmaps: Fix check for existing bitmaps

qemuMigrationDstPrepareAnyBlockDirtyBitmaps: Fix check for existing bitmaps

On incoming migration qemu doesn't load bitmaps into memory (which makes
them available under the 'dirty-bitmaps' field which we parse as the
'bitmaps' array in 'qemuBlockNamedNodeData') until after actually
resuming CPUs, thus the check for existing bitmaps never actually
worked.

We need to check the 'qcow2bitmaps' field instead which is populated
from the qcow2 headers prior to activating the image.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>