Detected by valgrind. Leaks introduced in commit 4d5383fd.

* tools/virsh.c: fix memory leaks on cmdDomXMLFromNative and cmdDomXMLToNative.

* how to reproduce?

  % virsh dumpxml ${guest} > foo.xml
  % valgrind -v --leak-check=full virsh domxml-from-native qemu-argv foo.xml
  % valgrind -v --leak-check=full virsh domxml-to-native qemu-argv foo.xml

* actual valgrind results:

==9724== 8,193 bytes in 1 blocks are definitely lost in loss record 31 of 33
==9724==    at 0x4A05FDE: malloc (vg_replace_malloc.c:236)
==9724==    by 0x4A06167: realloc (vg_replace_malloc.c:525)
==9724==    by 0x4C7510B: virReallocN (memory.c:161)
==9724==    by 0x4C84679: virFileReadLimFD (util.c:394)
==9724==    by 0x4C84815: virFileReadAll (util.c:455)
==9724==    by 0x41A89F: cmdDomXMLFromNative (virsh.c:5532)
==9724==    by 0x414872: vshCommandRun (virsh.c:16464)
==9724==    by 0x425623: main (virsh.c:17971)
==9724==
==9724== LEAK SUMMARY:
==9724==    definitely lost: 8,193 bytes in 1 blocks
==9724==    indirectly lost: 0 bytes in 0 blocks
==9724==      possibly lost: 0 bytes in 0 blocks
==9724==    still reachable: 127,128 bytes in 1,347 blocks

==7409== 8,193 bytes in 1 blocks are definitely lost in loss record 31 of 33
==7409==    at 0x4A05FDE: malloc (vg_replace_malloc.c:236)
==7409==    by 0x4A06167: realloc (vg_replace_malloc.c:525)
==7409==    by 0x4C7510B: virReallocN (memory.c:161)
==7409==    by 0x4C84679: virFileReadLimFD (util.c:394)
==7409==    by 0x4C84815: virFileReadAll (util.c:455)
==7409==    by 0x41A7AF: cmdDomXMLToNative (virsh.c:5578)
==7409==    by 0x414892: vshCommandRun (virsh.c:16463)
==7409==    by 0x425633: main (virsh.c:17970)
==7409==
==7409== LEAK SUMMARY:
==7409==    definitely lost: 8,193 bytes in 1 blocks
==7409==    indirectly lost: 0 bytes in 0 blocks
==7409==      possibly lost: 0 bytes in 0 blocks
==7409==    still reachable: 127,128 bytes in 1,347 blocks
Signed-off-by: NAlex Jia <ajia@redhat.com>