block: Update BlockDriverState.inherits_from on bdrv_set_backing_hd()

block: Update BlockDriverState.inherits_from on bdrv_set_backing_hd()

When a BlockDriverState's child is opened (be it a backing file, the
protocol layer, or any other) inherits_from is set to point to the
parent node. Children opened separately and then attached to a parent
don't have this pointer set.

bdrv_reopen_queue_child() uses this to determine whether a node's
children must also be reopened inheriting the options from the parent
or not. If inherits_from points to the parent then the child is
reopened and its options can be changed, like in this example:

   $ qemu-img create -f qcow2 hd0.qcow2 1M
   $ qemu-img create -f qcow2 hd1.qcow2 1M
   $ $QEMU -drive if=none,node-name=hd0,file=hd0.qcow2,\
                  backing.driver=qcow2,backing.file.filename=hd1.qcow2
   (qemu) qemu-io hd0 "reopen -o backing.l2-cache-size=2M"

If the child does not inherit from the parent then it does not get
reopened and its options cannot be changed:

   $ $QEMU -drive if=none,node-name=hd1,file=hd1.qcow2
           -drive if=none,node-name=hd0,file=hd0.qcow2,backing=hd1
   (qemu) qemu-io hd0 "reopen -o backing.l2-cache-size=2M"
   Cannot change the option 'backing.l2-cache-size'

If a disk image has a chain of backing files then all of them are also
connected through their inherits_from pointers (i.e. it's possible to
walk the chain in reverse order from base to top).

However this is broken if the intermediate nodes are removed using
e.g. block-stream because the inherits_from pointer from the base node
becomes NULL:

   $ qemu-img create -f qcow2 hd0.qcow2 1M
   $ qemu-img create -f qcow2 -b hd0.qcow2 hd1.qcow2
   $ qemu-img create -f qcow2 -b hd1.qcow2 hd2.qcow2
   $ $QEMU -drive if=none,file=hd2.qcow2
   (qemu) qemu-io none0 "reopen -o backing.l2-cache-size=2M"
   (qemu) block_stream none0 0 hd0.qcow2
   (qemu) qemu-io none0 "reopen -o backing.l2-cache-size=2M"
   Cannot change the option 'backing.l2-cache-size'

This patch updates the inherits_from pointer if the intermediate nodes
of a backing chain are removed using bdrv_set_backing_hd(), and adds a
test case for this scenario.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>