virt: Filter internal data for getVMList API

Filter out all keys that begin with '_' when returning detailed VM
information via the getVMList API.  This is needed by live merge which
will persist block job information in a '_blockJobs' key.  We don't want
that data to be made public

Change-Id: I39ac6935e5b95912eed42f16871566722316515a
Signed-off-by: Adam Litke <alitke@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/28532
Reviewed-by: Federico Simoncelli <fsimonce@redhat.com>