python: avoid unlikely sign extension bug

python: avoid unlikely sign extension bug

Detected by Coverity.  cpumap was allocated with a value of
(unsigned short)*(int), which is an int computation, and then
promotes to size_t.  On a 64-bit platform, this fails if bit
32 of the product is set (because of sign extension giving
a HUGE value to malloc), even though a naive programmer would
assume that since the first value is unsigned, the product
is also unsigned and at most 4GB would be allocated.

Won't bite in practice (the product should never be that large),
but worth using the right types to begin with, so that we are
now computing (unsigned short)*(size_t).

* python/libvirt-override.c (libvirt_virDomainGetVcpus): Use
correct type.