memory: cleanup side effects of memory_region_init_foo() on failure

if MemoryRegion intialization fails it's left in semi-initialized state,
where it's size is not 0 and attached as child to owner object.
And this leds to crash in following use-case:
    (monitor) object_add memory-backend-file,id=mem1,size=99999G,mem-path=/tmp/foo,discard-data=yes
    memory.c:2083: memory_region_get_ram_ptr: Assertion `mr->ram_block' failed
    Aborted (core dumped)
it happens due to assumption that memory region is intialized when
   memory_region_size() != 0
and therefore it's ok to access it in
   file_backend_unparent()
      if (memory_region_size() != 0)
          memory_region_get_ram_ptr()

which happens when object_add fails and unparents failed backend making
file_backend_unparent() access invalid memory region.

Fix it by making sure that memory_region_init_foo() APIs cleanup externally
visible side effects on failure (like set size to 0 and unparenting object)

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Message-Id: <1536064777-42312-1-git-send-email-imammedo@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>