scripts/kvm/kvm_stat: Make tui function a class

scripts/kvm/kvm_stat: Make tui function a class

The tui function itself had a few sub-functions and therefore
basically already was class-like. Making it an actual one with proper
methods improved readability.

The curses wrapper was dropped in favour of __entry/exit__ methods
that implement the same behaviour.

Also renamed single character variable name, so the name reflects the
content.

Signed-off-by: Janosch Frank <frankja@linux.vnet.ibm.com>
Message-Id: <1452525484-32309-28-git-send-email-frankja@linux.vnet.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>