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

Skipped files count is available in CLI output of rebalance status
and remove-brick status, but not available in xml output.

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>
      <status>0</status>
      <statusStr>completed</statusStr>
    </node>
    <aggregate>
      <files>0</files>
      <size>0</size>
      <lookups>0</lookups>
      <failures>0</failures>
      <skipped>0</skipped>
      <status>0</status>
      <statusStr>completed</statusStr>
    </aggregate>
  </volRebalance>
</cliOutput>

BUG: 1012772
Change-Id: I4f5128c3fa3b2b791bcbaf41ac7f229f80bfefe9
Signed-off-by: Aravinda VK <avishwan@redhat.com>
Reviewed-on: http://review.gluster.org/6000
Reviewed-by: Kaushal M <kaushal@redhat.com>
Reviewed-on: http://review.gluster.org/6119
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>