qemu: snapshot: go through cleanup on error
qemu: snapshot: go through cleanup on error

A recent commit added an error check for too-nested backing chains
followed by a return, even though errors above jump to cleanup.

Signed-off-by: Ján Tomko <4cab11cfb98d3c937327354a78eb07dbb6ee2bc6@redhat.com>
Fixes: b168fa88b85dec181882816ab65a59a6c4500667
Reviewed-by: Daniel P. Berrangé <bb938cf255e055ff3507f2627d214e8e62118fcf@redhat.com>
Reviewed-by: Andrea Bolognani <3ada0bee826c753786fdbba72243dfba997094cf@redhat.com>

fabianfreyer/libvirt,jfehlig/libvirt,crobinso/libvirt,crobinso/libvirt,fabianfreyer/libvirt,nertpinx/libvirt,libvirt/libvirt,nertpinx/libvirt,fabianfreyer/libvirt,jardasgit/libvirt,olafhering/libvirt,jardasgit/libvirt,olafhering/libvirt,fabianfreyer/libvirt,libvirt/libvirt,nertpinx/libvirt,zippy2/libvirt,nertpinx/libvirt,zippy2/libvirt,olafhering/libvirt,crobinso/libvirt,jardasgit/libvirt,jardasgit/libvirt,libvirt/libvirt,jfehlig/libvirt,jardasgit/libvirt,olafhering/libvirt,libvirt/libvirt,zippy2/libvirt,fabianfreyer/libvirt,crobinso/libvirt,zippy2/libvirt,jfehlig/libvirt,nertpinx/libvirt,jfehlig/libvirt