qapi: Add optional field "name" to block dirty bitmap

qapi: Add optional field "name" to block dirty bitmap

This field will be set for user created dirty bitmap. Also pass in an
error pointer to bdrv_create_dirty_bitmap, so when a name is already
taken on this BDS, it can report an error message. This is not global
check, two BDSes can have dirty bitmap with a common name.

Implemented bdrv_find_dirty_bitmap to find a dirty bitmap by name, will
be used later when other QMP commands want to reference dirty bitmap by
name.

Add bdrv_dirty_bitmap_make_anon. This unsets the name of dirty bitmap.

Signed-off-by: Fam Zheng <[email protected]>
Signed-off-by: John Snow <[email protected]>
Reviewed-by: Max Reitz <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Reviewed-by: Stefan Hajnoczi <[email protected]>
Message-id: 1429314609[email protected]
Signed-off-by: Stefan Hajnoczi <[email protected]>
Signed-off-by: Kevin Wolf <[email protected]>