qemu: Use 'format' nodename accessors for block dirty bitmap operations

qemu: Use 'format' nodename accessors for block dirty bitmap operations

In most cases the bitmap operations are relevant only on qcow2 images
thus the 'format' layer will be present. Although in certain specific
cases temporary bitmaps can be created on top of other images as well,
thus we use the 'effective' bitmap name in all cases for bitmap
operations.

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