configure: Enable KVM for aarch64 host/target combination

Enable KVM if the host and target CPU are both aarch64. Note
that host aarch64 + target arm is not valid for KVM acceleration:
the 64 bit kernel does not support the ioctl interface for
32 bit CPUs. 32 bit VMs on 64 bit hosts need to be created
using the 64 bit ioctl interface; when QEMU supports this it
will be on the arch64-softmmu target with a -cpu parameter for
a 32 bit CPU, which is still an aarch64/aarch64 combination
as far as configure is concerned.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1385645602-18662-5-git-send-email-peter.maydell@linaro.org
Reviewed-by: Christoffer Dall <christoffer.dall@linaro.org>
