storage: do not leak storage pool XML filename

storage: do not leak storage pool XML filename

Valgrind complained:

==28277== 38 bytes in 1 blocks are definitely lost in loss record 298 of 957
==28277==    at 0x4A06A2E: malloc (vg_replace_malloc.c:270)
==28277==    by 0x82D7F57: __vasprintf_chk (in /lib64/libc-2.12.so)
==28277==    by 0x52EF16A: virVasprintfInternal (stdio2.h:199)
==28277==    by 0x52EF25C: virAsprintfInternal (virstring.c:514)
==28277==    by 0x52B1FA9: virFileBuildPath (virfile.c:2831)
==28277==    by 0x19B1947C: storageDriverAutostart (storage_driver.c:191)
==28277==    by 0x19B196A7: storageStateAutoStart (storage_driver.c:307)
==28277==    by 0x538527E: virStateInitialize (libvirt.c:793)
==28277==    by 0x11D7CF: daemonRunStateInit (libvirtd.c:947)
==28277==    by 0x52F4694: virThreadHelper (virthread.c:206)
==28277==    by 0x6E08A50: start_thread (in /lib64/libpthread-2.12.so)
==28277==    by 0x82BE93C: clone (in /lib64/libc-2.12.so)

Signed-off-by: Michael Chapman <mike@very.puzzling.org>