Use '==' for equality checks and 'is' when comparing identities.

An example where '==' and 'is' behave differently:
>>> a = 4242
>>> a == 4242
True
>>> a is 4242
False

Signed-off-by: Marc Hartmayer <mhartmay@linux.vnet.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>