scripts/kvm/kvm_stat: Group arch specific data

Using global variables and multiple initialization functions for arch
specific data makes the code hard to read. By grouping them in the
Arch classes we encapsulate and initialize them in one place.

Signed-off-by: Janosch Frank <frankja@linux.vnet.ibm.com>
Message-Id: <1452525484-32309-26-git-send-email-frankja@linux.vnet.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
