python: cleanup vcpu related binding APIs

python: cleanup vcpu related binding APIs

libvirt_virDomainGetVcpus: add error handling, return -1 instead of None
libvirt_virDomainPinVcpu and libvirt_virDomainPinVcpuFlags:
  check the type of argument
  make use of libvirt_boolUnwrap

  Set bitmap according to these values which are contained in given
  argument of vcpu tuple and turn off these bit corresponding to
  missing vcpus in argument tuple

  The original way ignored the error info from PyTuple_GetItem
  if index is out of range.
  "IndexError: tuple index out of range"
  The error message will only be raised on next command in interactive mode.