virsh: avoid memory leak on cmdVolCreateAs

virsh: avoid memory leak on cmdVolCreateAs

* tools/virsh.c: fix memory leak on cmdVolCreateAs function.

* Detected in valgrind run:

==4746==
==4746== 48 (40 direct, 8 indirect) bytes in 1 blocks are definitely lost in loss record 26 of 52
==4746==    at 0x4A04A28: calloc (vg_replace_malloc.c:467)
==4746==    by 0x4C76E51: virAlloc (memory.c:101)
==4746==    by 0x4CD9418: virGetStoragePool (datatypes.c:592)
==4746==    by 0x4D21367: remoteStoragePoolLookupByName (remote_driver.c:4126)
==4746==    by 0x4CE42B0: virStoragePoolLookupByName (libvirt.c:10232)
==4746==    by 0x40C276: vshCommandOptPoolBy (virsh.c:13660)
==4746==    by 0x40CA37: cmdVolCreateAs (virsh.c:8094)
==4746==    by 0x412AF2: vshCommandRun (virsh.c:13770)
==4746==    by 0x422F11: main (virsh.c:15127)
==4746==
==4746== 1,011 bytes in 1 blocks are definitely lost in loss record 45 of 52
==4746==    at 0x4A05FDE: malloc (vg_replace_malloc.c:236)
==4746==    by 0x4A06167: realloc (vg_replace_malloc.c:525)
==4746==    by 0x4C76ECB: virReallocN (memory.c:161)
==4746==    by 0x4C60319: virBufferGrow (buf.c:72)
==4746==    by 0x4C606AA: virBufferAdd (buf.c:106)
==4746==    by 0x40CB37: cmdVolCreateAs (virsh.c:8118)
==4746==    by 0x412AF2: vshCommandRun (virsh.c:13770)
==4746==    by 0x422F11: main (virsh.c:15127)
==4746==
==4746== LEAK SUMMARY:
==4746==    definitely lost: 1,051 bytes in 2 blocks
==4746==    indirectly lost: 8 bytes in 1 blocks
==4746==      possibly lost: 0 bytes in 0 blocks
==4746==    still reachable: 390,767 bytes in 1,373 blocks
==4746==         suppressed: 0 bytes in 0 blocks

* How to reproduce?

% valgrind -v --leak-check=full virsh vol-create-as default foo.img 10M \
  --allocation 0 --format qcow2 --backing-vol bar.img

Notes: bar.img doesn't exist.

Signed-off-by: Alex Jia <ajia@redhat.com>