qemu: roll back if not all nbdkit backends are successful

qemu: roll back if not all nbdkit backends are successful

When starting nbdkit processes for the backing store of a disk, we were
returning an error if any backing store failed, but we were not cleaning
up processes that succeeded higher in the chain. Make sure that if we
return a failure status from qemuNbdkitStartStorageSource() that we roll
back any processes that had been started.

Signed-off-by: Jonathon Jongsma <jjongsma@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>