scripts/kvm/kvm_stat: Cleanup of platform detection

s390 machines can also be detected via uname -m, i.e. python's
os.uname, no need for more complicated checks.

Calling uname once and saving its value for multiple checks is
perfectly sufficient. We don't expect the machine's architecture to
change when the script is running anyway.

On multi-cpu systems x86_init currently will get called multiple
times, returning makes sure we don't waste cicles on that.

Signed-off-by: Janosch Frank <frankja@linux.vnet.ibm.com>
Message-Id: <1452525484-32309-16-git-send-email-frankja@linux.vnet.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
