Revert "qemu: Don't repeat virDomainObjEndAPI in qemuDomainBlockPull"

Revert "qemu: Don't repeat virDomainObjEndAPI in qemuDomainBlockPull"

This reverts commit 421c9550f5446729b513ee50f5c44e6f6969b5a2

qemuDomainBlockPullCommon calls virDomainObjEndAPI internally so the
original commit made us shed two references of @vm instead of one
getting us into a premature free of @vm.

This is not a straight revert as qemuDomainBlockPull was modified
meanwhile. I've also added a warning comment that @vm is consumed.

https://bugzilla.redhat.com/show_bug.cgi?id=1777230

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>