Fix dereference of potentially freed pointer in qemudDomainSaveFlags

Fix dereference of potentially freed pointer in qemudDomainSaveFlags

The pointer to the xml describing the domain is saved into an object
prior to calling VIR_REALLOC_N() to make the size of the memory it
points to a multiple of QEMU_MONITOR_MIGRATE_TO_FILE_BS. If that
operation needs to allocate new memory, the pointer that was saved is
no longer valid.

To avoid this situation, adjust the size *before* saving the pointer.

(This showed up when experimenting with very large values of
QEMU_MONITOR_MIGRATE_TO_FILE_BS).