qemuxml2argvtest: Don't leak dummy monitor

qemuxml2argvtest: Don't leak dummy monitor

It's just test, but why leak it?

==26971== 20 bytes in 1 blocks are definitely lost in loss record 623 of 704
==26971==    at 0x4C29F80: malloc (vg_replace_malloc.c:296)
==26971==    by 0xE560447: vasprintf (vasprintf.c:76)
==26971==    by 0xAE0DEE2: virVasprintfInternal (virstring.c:480)
==26971==    by 0xAE0DFF7: virAsprintfInternal (virstring.c:501)
==26971==    by 0x4751F3: qemuProcessPrepareMonitorChr (qemu_process.c:2651)
==26971==    by 0x4334B1: testCompareXMLToArgvFiles (qemuxml2argvtest.c:297)
==26971==    by 0x4339AC: testCompareXMLToArgvHelper (qemuxml2argvtest.c:413)
==26971==    by 0x446E7A: virTestRun (testutils.c:179)
==26971==    by 0x445D33: mymain (qemuxml2argvtest.c:2029)
==26971==    by 0x44886F: virTestMain (testutils.c:969)
==26971==    by 0x445D9B: main (qemuxml2argvtest.c:2036)

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