fix patch for accepting NULL qiov when padding

fix patch for accepting NULL qiov when padding

All callers of the function pass an address, so dereferencing once
before checking for NULL is required. It's also necessary to update
bytes and offset nevertheless, so the request will actually be aligned
later and not trigger an assertion failure.

Seems like this was accidentally broken in 8dca018 ("udpate and rebase
to QEMU v6.0.0") and this is effectively a revert to the original
version of the patch. The qiov functions changed back then, which
might've been the reason Stefan tried to simplify the patch.

Should fix live-import for certain kinds of VMDK images.

Reported-by: Wolfgang Bumiller <w.bumiller@proxmox.com>
Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>