The "redundancy" option for Sheepdog image creation is currently a
string that can encode one or two integers depending on its format,
which at the same time implicitly selects a mode.
This patch turns it into a QAPI union and converts the string into such
a QAPI object before interpreting the values.
Signed-off-by: Kevin Wolf <kwolf@redhat.com> Reviewed-by: Max Reitz <mreitz@redhat.com>