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
3bb28b7208b349e7a1b326e3c6ef9efac1d462bf?
    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: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
