KVM: selftests: test behavior of unmapped L2 APIC-access address

Add a regression test for commit 671ddc70 ("KVM: nVMX: Don't leak
L1 MMIO regions to L2").

First, check to see that an L2 guest can be launched with a valid
APIC-access address that is backed by a page of L1 physical memory.

Next, set the APIC-access address to a (valid) L1 physical address
that is not backed by memory. KVM can't handle this situation, so
resuming L2 should result in a KVM exit for internal error
(emulation).

Signed-off-by: Jim Mattson <jmattson@google.com>
Reviewed-by: Ricardo Koller <ricarkol@google.com>
Reviewed-by: Peter Shier <pshier@google.com>
Message-Id: <20201026180922.3120555-1-jmattson@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>