libxlDriverConfigDispose: Free @configBaseDir too

libxlDriverConfigDispose: Free @configBaseDir too

Allocated in libxlDriverConfigNew(), the @configBaseDir is never
freed.

 13 bytes in 1 blocks are definitely lost in loss record 36 of 125
    at 0x483579F: malloc (vg_replace_malloc.c:299)
    by 0x8012469: strdup (strdup.c:42)
    by 0x52926DE: virStrdup (virstring.c:966)
    by 0x11D46B: libxlDriverConfigNew (libxl_conf.c:1749)
    by 0x114D78: testCompareXMLToDomConfig (libxlxml2domconfigtest.c:62)
    by 0x1152A3: testCompareXMLToDomConfigHelper (libxlxml2domconfigtest.c:160)
    by 0x115925: virTestRun (testutils.c:174)
    by 0x1154A4: mymain (libxlxml2domconfigtest.c:216)
    by 0x1179E9: virTestMain (testutils.c:1096)
    by 0x1154FD: main (libxlxml2domconfigtest.c:224)

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>