virsh: avoid double free of domain

virsh: avoid double free of domain

* tools/virsh.c: avoid double free of domain, when weight value of blkiotune
  less than 0, codes will free domain and jump to cleanup section, however,
  cleanup will free domain again.

Detected in valgrind run:

==21297== ERROR SUMMARY: 20 errors from 20 contexts (suppressed: 69 from 8)
==21297==
==21297== 1 errors in context 1 of 20:
==21297== Invalid read of size 4
==21297==    at 0x40E209B: virDomainFree (libvirt.c:2096)
==21297==    by 0x8065274: cmdBlkiotune (virsh.c:3695)
==21297==    by 0x8054CC1: vshCommandRun (virsh.c:13135)
==21297==    by 0x806B967: main (virsh.c:14487)
==21297==  Address 0x446ad48 is 0 bytes inside a block of size 36 free'd
==21297==    at 0x4005B0A: free (vg_replace_malloc.c:325)
==21297==    by 0x406814D: virFree (memory.c:310)
==21297==    by 0x40D6635: virReleaseDomain (datatypes.c:243)
==21297==    by 0x40D6C5E: virUnrefDomain (datatypes.c:280)
==21297==    by 0x40E20B9: virDomainFree (libvirt.c:2101)
==21297==    by 0x8065297: cmdBlkiotune (virsh.c:3613)
==21297==    by 0x8054CC1: vshCommandRun (virsh.c:13135)
==21297==    by 0x806B967: main (virsh.c:14487)
==21297==
==21297==
==21297== 1 errors in context 2 of 20:
==21297== Invalid read of size 4
==21297==    at 0x40E1FE6: virDomainFree (libvirt.c:2092)
==21297==    by 0x8065274: cmdBlkiotune (virsh.c:3695)
==21297==    by 0x8054CC1: vshCommandRun (virsh.c:13135)
==21297==    by 0x806B967: main (virsh.c:14487)
==21297==  Address 0x446ad48 is 0 bytes inside a block of size 36 free'd
==21297==    at 0x4005B0A: free (vg_replace_malloc.c:325)
==21297==    by 0x406814D: virFree (memory.c:310)
==21297==    by 0x40D6635: virReleaseDomain (datatypes.c:243)
==21297==    by 0x40D6C5E: virUnrefDomain (datatypes.c:280)
==21297==    by 0x40E20B9: virDomainFree (libvirt.c:2101)
==21297==    by 0x8065297: cmdBlkiotune (virsh.c:3613)
==21297==    by 0x8054CC1: vshCommandRun (virsh.c:13135)
==21297==    by 0x806B967: main (virsh.c:14487)

* how to reproduce?
  % valgrind -v --leak-check=full virsh blkiotune guestname --weight -1