qemu-img: Resolve relative backing paths in rebase
    
    Currently, rebase interprets a relative path for the new backing image
    as follows:
    (1) Open the new backing image with the given relative path (thus relative to
        qemu-img's working directory).
    (2) Write it directly into the overlay's backing path field (thus
        relative to the overlay).
    
    If the overlay is not in qemu-img's working directory, both will be
    different interpretations, which may either lead to an error somewhere
    (either rebase fails because it cannot open the new backing image, or
    your overlay becomes unusable because its backing path does not point to
    a file), or, even worse, it may result in your rebase being performed
    for a different backing file than what your overlay will point to after
    the rebase.
    
    Fix this by interpreting the target backing path as relative to the
    overlay, like qemu-img does everywhere else.
    
    Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=1569835
    Cc: qemu-stable@nongnu.org
    Signed-off-by: Max Reitz <mreitz@redhat.com>
    Message-id: 20180509182002.8044-2-mreitz@redhat.com
    Reviewed-by: Eric Blake <eblake@redhat.com>
    Signed-off-by: Max Reitz <mreitz@redhat.com>
    (cherry picked from commit d16699b64671466b42079c45b89127aeea1ca565)
    Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>