Even though Coverity can prove that 'last' is always set if the prior
loop executed, gcc 8.0.1 cannot:

  CC       conf/libvirt_conf_la-virdomainmomentobjlist.lo
../../src/conf/virdomainmomentobjlist.c: In function 'virDomainMomentMoveChildren':
../../src/conf/virdomainmomentobjlist.c:178:19: error: 'last' may be used uninitialized in this function [-Werror=maybe-uninitialized]
         last->sibling = to->first_child;
         ~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~

Rewrite the loop to a form that should be easier for static analysis
to work with.

Fixes: ced0898fReported-by: NBjoern Walk <bwalk@linux.ibm.com>
Signed-off-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>