qemu: re-add padding to the saved state images

In the past we added 1024 bytes of padding to saved state images so that
users can run "virsh managedsave-edit $GUEST" and make XML changes which
increase the size of the XML document. This padding was accidentally
lost a while back

commit 6b9b21db7079888a05d192b079e68290bdf14a76
Author: Peter Krempa <pkrempa@redhat.com>
Date: Wed Feb 17 13:10:11 2016 +0100

qemu: Remove unnecessary calculations in qemuDomainSaveMemory

The original 1024 bytes was unreasonably stingy when we consider that
the QEMU state is typically going to be many 100's of MB in size. Thus
this adds 64 KB of padding after the XML which should cope with any
plausible modifications a user will want to make.

https://bugzilla.redhat.com/show_bug.cgi?id=1229255

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>