virDomainSnapshotAlignDisks: clarify handing of snapshot location
virDomainSnapshotAlignDisks: clarify handing of snapshot location

Remove the use of the 'disk_snapshot' temporary variable since accessing
the disk definition now isn't that much longer to write and use explicit
value checks instead of the (non-)zero check to make it more obvious
what the code is doing.

Signed-off-by: Peter Krempa <2cf5c04c61aa466e4a47bfedc747d17279c72ffc@redhat.com>
Reviewed-by: Ján Tomko <4cab11cfb98d3c937327354a78eb07dbb6ee2bc6@redhat.com>
Reviewed-by: Eric Blake <a9da18479c871fe903d9590822cfe2629c6a4d11@redhat.com>

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