Add comma escaping for netsource. This is done here because
qemuBuildNetworkDriveStr has other external callers which
may not expect an escaped comma; however, this particular
command building path needs to perform the escaping for the
hostdev command line, so we do it now to ensure src->path
and src->host->name are covered.
Signed-off-by: NAnya Harter <aharter@redhat.com>