zeroinit: fix regression with filename parsing

zeroinit: fix regression with filename parsing

As reported in the community forum [0], cloning or importing images
to RBD storages (without the krbd setting) was broken. This is a
result of no filename parsing happening anymore in bdrv_open_child()
after commit b242e7f ("backport fix for CVE-2024-4467"), which the
zeroinit relied on for passing along the RBD filename+key-value pairs.

There is a dedicated function for opening the file child which still
does filename parsing. Use that for opening the file child. Role and
flags should still be the same as with the manual bdrv_open_child(),
because the zeroinit driver is a filter, and the assignment bs->file
is also done by bdrv_open_file_child().

Fixes: b242e7f ("backport fix for CVE-2024-4467")
Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
[0]: https://forum.proxmox.com/threads/qemu-9-0-available-on-pve-no-subscription-as-of-now.149772/post-681620
FG: added missing link

Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>