block: Relative backing file for image creation

block: Relative backing file for image creation

Relative backing filenames are always relative to the backed image's
directory; the same applies to image creation. Therefore, if the backing
file has to be opened for determining its size (in case the size has not
been explicitly specified) its filename should be interpreted relative
to the new image's base directory and not relative to qemu's working
directory.

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