qemuStateInitialize: Don't leak @memoryBackingPath

qemuStateInitialize: Don't leak @memoryBackingPath

==899== 39 bytes in 1 blocks are definitely lost in loss record 732 of 1,003
==899==    at 0x4C2AEDF: malloc (vg_replace_malloc.c:299)
==899==    by 0x8B68CE7: vasprintf (in /lib64/libc-2.25.so)
==899==    by 0x55498D2: virVasprintfInternal (virstring.c:708)
==899==    by 0x55499E7: virAsprintfInternal (virstring.c:729)
==899==    by 0x2BECFFF0: qemuGetMemoryBackingBasePath (qemu_conf.c:1757)
==899==    by 0x2BF23225: qemuStateInitialize (qemu_driver.c:893)
==899==    by 0x563073D: virStateInitialize (libvirt.c:770)
==899==    by 0x124CC4: daemonRunStateInit (libvirtd.c:834)
==899==    by 0x55521CD: virThreadHelper (virthread.c:206)
==899==    by 0x88D9686: start_thread (in /lib64/libpthread-2.25.so)
==899==    by 0x8BEAEFE: clone (in /lib64/libc-2.25.so)

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