memory_region_present: return false if address is not found in child MemoryRegion

Windows XP shows COM2 port as non functional in
"Device Manager" although no COM2 port backing device
is present in QEMU.

This regression is really due to
3bb28b72?
    memory: Provide separate handling of unassigned io ports accesses

That is caused by the fact that QEMU reports to
OSPM that device is present by setting 5th bit in
PII4XPM.pci_conf[0x67] register when COM2 doesn't
exist.

It happens due to memory_region_present(io_as, 0x2f8)
returning false positive since 0x2f8 address eventually
translates into catchall io_as address space.

Fix memory_region_present(parent, addr) by returning
true only if addr maps into a MemoryRegion within
parent (excluding parent itself), to match its
doc comment.

While at it fix copy/paste error in
memory_region_present() doc comment.

Note: this is a temporary hack: we really need better handling for
unassigned regions, we should avoid fallback regions since they are bad
for performance (breaking radix tree assumption that the data structure
is sparsely populated); for memory we need to fix this to implement PCI
master abort properly, anyway.

Cc: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: NIgor Mammedov <imammedo@redhat.com>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

include/exec/memory.h

@@ -836,13 +836,13 @@ void memory_region_set_alias_offset(MemoryRegion *mr,
                                     hwaddr offset);
 
 /**
- * memory_region_present: translate an address/size relative to a
- * MemoryRegion into a #MemoryRegionSection.
+ * memory_region_present: checks if an address relative to a @parent
+ * translates into #MemoryRegion within @parent
  *
  * Answer whether a #MemoryRegion within @parent covers the address
  * @addr.
  *
- * @parent: a MemoryRegion within which @addr is a relative address
+ * @parent: a #MemoryRegion within which @addr is a relative address
  * @addr: the area within @parent to be searched
  */
 bool memory_region_present(MemoryRegion *parent, hwaddr addr);

memory.c

@@ -1562,7 +1562,7 @@ static FlatRange *flatview_lookup(FlatView *view, AddrRange addr)
 bool memory_region_present(MemoryRegion *parent, hwaddr addr)
 {
     MemoryRegion *mr = memory_region_find(parent, addr, 1).mr;
-    if (!mr) {
+    if (!mr || (mr == parent)) {
         return false;
     }
     memory_region_unref(mr);