domain_conf: Free egl render node in virDomainGraphicsDefFree

domain_conf: Free egl render node in virDomainGraphicsDefFree

 13 bytes in 1 blocks are definitely lost in loss record 44 of 179
    at 0x4C2EE6F: malloc (vg_replace_malloc.c:299)
    by 0x9514A69: strdup (in /lib64/libc-2.27.so)
    by 0x5E60C0B: virStrdup (virstring.c:956)
    by 0x54C856F: virHostGetDRMRenderNode (qemuxml2argvmock.c:190)
    by 0x57CB4E3: qemuProcessGraphicsSetupRenderNode (qemu_process.c:4860)
    by 0x57CB571: qemuProcessSetupGraphics (qemu_process.c:4881)
    by 0x57CE01B: qemuProcessPrepareDomain (qemu_process.c:6040)
    by 0x57D102E: qemuProcessCreatePretendCmd (qemu_process.c:6975)
    by 0x114C1C: testCompareXMLToArgv (qemuxml2argvtest.c:611)
    by 0x134B90: virTestRun (testutils.c:174)
    by 0x123478: mymain (qemuxml2argvtest.c:1697)
    by 0x136BFA: virTestMain (testutils.c:1112)

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Erik Skultety <eskultet@redhat.com>