arm, kvm: enable running 32-bit Guest under ARM KVM64

arm, kvm: enable running 32-bit Guest under ARM KVM64

1) Pass KVM_ARM_VCPU_EL1_32BIT to kvmArmVCpuInit
   when running 32-bit OS

2) Correctly map 64-bit registers to banked 32-bit ones

Change-Id: I1dec6427d6f5c3bba599ccdd804f1dfe80d3e670
Reviewed-on: https://gem5-review.googlesource.com/2261
Maintainer: Rahul Thakur <rjthakur@google.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>