block: Get full backing filename from string

block: Get full backing filename from string

Introduce bdrv_get_full_backing_filename_from_filename(), a function
which takes the name of the backed file and a potentially relative
backing filename to produce the full (absolute) backing filename.

Use this function from bdrv_get_full_backing_filename().

Signed-off-by: Max Reitz <[email protected]>
Reviewed-by: Kevin Wolf <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Reviewed-by: Fam Zheng <[email protected]>
Signed-off-by: Kevin Wolf <[email protected]>