A couple of allocation were not calling virReportOOMError on allocation
errors
* src/util/hash.c: add the needed call in virHashCreate and
virHashAddOrUpdateEntry
Most of the hash iterators need to modify either payload of
data args. The const annotation prevents this.
* src/util/hash.h, src/util/hash.c: Remove const-ness from
virHashForEach/Iterator
* src/xen/xm_internal.c: Remove bogus casts