common: admin sock output XML elements whitespace

Replace white space with underscore in XML elements displayed from admin
socket commands. <config get>...</config get> becomes
<config_get>...</config_get>

http://tracker.ceph.com/issues/10199 Fixes: #10199

Signed-off-by: Loic Dachary <ldachary@redhat.com>
Merge pull request #3031 from dachary/wip-10199-config-get-xml

common: admin sock output XML elements whitespace

Reviewed-by: Sahid Orentino Ferdjaoui <sahid.ferdjaoui@redhat.com>