x86: realmode: initialize idtr

The realmode test does not initialize the IDTR, assuming that its base
is zero and its limit 0x3ff. Initialize it, as the bootloader might not
set it as such.

Signed-off-by: Nadav Amit <namit@vmware.com>
Message-Id: <20200626092333.2830-2-namit@vmware.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>