New remote command for exporting a single domain

The command is used as follows:

    getDomainWithSettingsXML <domain>

and outputs the requested domain to the standard output.

In that end, the const version of CConfigurableDomains::findConfigurableDomain
is made public. This somewhat breaks encapsulation but since the returned
domain is const, this has limited impact.

Change-Id: I9b854040a5d59913b9b405c8e38d29a3018a6079
Signed-off-by: David Wagner <david.wagner@intel.com>