I am not sure if I understand your problem. You can easily specify the exact location of xml file, both as relative and fully qualified path.
<suiteXmlFile>c:/some/dir/testsuite-persistence-layer.xml</suiteXmlFile>
or
<suiteXmlFile>src/test/java/com/something/project/testsuite-persistence-layer.xml</suiteXmlFile>
But that's too easy, so I am guessing you are looking for a way to parametrize the directory where xmls are located. The quick solution that comes to my mind would be
<suiteXmlFile>${xmlPath}/testSuite.xml</suiteXmlFile>
Now you can run
mvn test -DxmlPath=c:/some/path
Of course xmlPath is just made-up name, you can use any other variable name you wish.
If you don't want to pass the path as an argument from command line you can specify the value of xmlPath variable in properties section of your POM. Properties is one of the main sections located just under < project > branch.
<project ... >
...
<properties>
<xmlPath>c:/some/path</xmlPath>
</properties>
...
<build>
...
<plugins>
...
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-surefire-plugin</artifactId>
<version>2.9</version>
<configuration>
<suiteXmlFiles>
<suiteXmlFile>${xmlPath}/testSuite.xml</suiteXmlFile>
</suiteXmlFiles>
...
</configuration>
</plugin>
...
</plugins>
...
</build>
...
</project>