qemu: snapshot: Setup disks for manual snapshot only when the VM is actually paused

qemu: snapshot: Setup disks for manual snapshot only when the VM is actually paused

When creating a snapshot with 'VIR_DOMAIN_SNAPSHOT_CREATE_LIVE' the VM
is paused only after dumping the memory state.

This means that also the steps to do a 'manual' disk snapshot
(deactivation of the block nodes in qemu) must happen only once the VM
is paused.

Move the manual snapshot setup code after the memory snapshot code.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>