virsh: edit: don't leak XML string on reedit or redefine

Free the old XML strings before overwriting them if the user
has chosen to reedit the file or force the redefinition.

Found by Alex Jia trying to reproduce another bug:
https://bugzilla.redhat.com/show_bug.cgi?id=977430#c3
(cherry picked from commit 1e3a252974c8e5c650f1d84dc2b167f0ae8cee3c)
Plug leak in virCgroupMoveTask

We only break out of the while loop if *content is an empty string.
However the buffer has been allocated to BUFSIZ + 1 (8193 in my case),
but it gets overwritten in the next for iteration.

Move VIR_FREE right before we overwrite it to avoid the leak.

==5777== 16,386 bytes in 2 blocks are definitely lost in loss record 1,022 of 1,027
==5777==    by 0x5296E28: virReallocN (viralloc.c:184)
==5777==    by 0x52B0C66: virFileReadLimFD (virfile.c:1137)
==5777==    by 0x52B0E1A: virFileReadAll (virfile.c:1199)
==5777==    by 0x529B092: virCgroupGetValueStr (vircgroup.c:534)
==5777==    by 0x529AF64: virCgroupMoveTask (vircgroup.c:1079)

Introduced by 83e4c77.

https://bugzilla.redhat.com/show_bug.cgi?id=978352
(cherry picked from commit 5bc8ecb8d1170f41d4c177c1cf0e87c54194a3a3)
Fix invalid read in virCgroupGetValueStr

Don't check for '\n' at the end of file if zero bytes were read.

Found by valgrind:
==404== Invalid read of size 1
==404==    at 0x529B09F: virCgroupGetValueStr (vircgroup.c:540)
==404==    by 0x529AF64: virCgroupMoveTask (vircgroup.c:1079)
==404==    by 0x1EB475: qemuSetupCgroupForEmulator (qemu_cgroup.c:1061)
==404==    by 0x1D9489: qemuProcessStart (qemu_process.c:3801)
==404==    by 0x18557E: qemuDomainObjStart (qemu_driver.c:5787)
==404==    by 0x190FA4: qemuDomainCreateWithFlags (qemu_driver.c:5839)

Introduced by 0d0b409.

https://bugzilla.redhat.com/show_bug.cgi?id=978356
(cherry picked from commit 306c49ffd56a1c72b1892d50f2a75531c62f4a1d)