Revert "memory: Return -1 again on reads from unsigned regions"

Revert "memory: Return -1 again on reads from unsigned regions"

This reverts commit 9b8c69243585a32d14b9bb9fcd52c37b0b5a1b71.

The commit was wrong: We only return -1 on invalid accesses, not on
valid but unbacked ones. This broke various corner cases.

Cc: qemu-stable@nongnu.org
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
(cherry picked from commit 68a7439a150d6b4da99082ab454b9328b151bc25)

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>