target-arm: Add minimal KVM AArch64 support

target-arm: Add minimal KVM AArch64 support

Add the bare minimum set of functions needed for control of an
AArch64 KVM vcpu:
 * CPU initialization
 * minimal get/put register functions which only handle the
   basic state of the CPU

Signed-off-by: Mian M. Hamayun <m.hamayun@virtualopensystems.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1385645602-18662-4-git-send-email-peter.maydell@linaro.org
[PMM: significantly overhauled; most notably:
 * code lives in kvm64.c rather than using #ifdefs
 * support '-cpu host' rather than implicitly using whatever the
   host's CPU is regardless of what the user requests
 * fix bug attempting to get/set nonexistent X[31]
 * fix bug writing 64 bit kernel pstate into uint32_t env field
]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Christoffer Dall <christoffer.dall@linaro.org>