scripts/kvm/kvm_stat: Fixup syscall error reporting

scripts/kvm/kvm_stat: Fixup syscall error reporting

In 2008 a patch was written that introduced ctypes.get_errno() and
set_errno() as official interfaces to the libc errno variable. Using
them we can avoid accessing private libc variables.
The patch was included in python 2.6.

Also we need to raise the right exception, with the right parameters
and a helpful message.

Signed-off-by: Janosch Frank <frankja@linux.vnet.ibm.com>
Message-Id: <1452525484-32309-14-git-send-email-frankja@linux.vnet.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>