cli: runtime in xml output of rebalance/remove-brick status

"runtime in secs" is available in the CLI output of
rebalance status and remove-brick status, but not available
in xml output when --xml is passed.

runtime in aggregate section will be max of all nodes runtimes.

Example output:
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<cliOutput>
  <opRet>0</opRet>
  <opErrno>0</opErrno>
  <opErrstr/>
  <volRebalance>
    <op>3</op>
    <nodeCount>1</nodeCount>
    <node>
      <nodeName>localhost</nodeName>
      <files>0</files>
      <size>0</size>
      <lookups>0</lookups>
      <failures>0</failures>
      <skipped>0</skipped>
      <runtime>1.00</runtime>
      <status>3</status>
      <statusStr>completed</statusStr>
    </node>
    <aggregate>
      <files>0</files>
      <size>0</size>
      <lookups>0</lookups>
      <failures>0</failures>
      <skipped>0</skipped>
      <runtime>1.00</runtime>
      <status>3</status>
      <statusStr>completed</statusStr>
    </aggregate>
  </volRebalance>
</cliOutput>

BUG: 1012773
Change-Id: I8deaba08922a53cd2d3b411e097a7b3cf591b127
Signed-off-by: Aravinda VK <avishwan@redhat.com>
Reviewed-on: http://review.gluster.org/5997
Reviewed-by: Kaushal M <kaushal@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>