ppc: add helpful message when KVM fails to start VCPU

On POWER8 systems, KVM checks if VCPU is running on primary threads,
and that secondary threads are offline. If this is not the case,
ioctl() fails with errno set to EBUSY.

QEMU aborts with a non explicit error message:
$ ./qemu-system-ppc64 --nographic -machine pseries,accel=kvm
error: kvm run failed Device or resource busy

To help user to diagnose the problem, this patch adds an informative
error message.

There is no easy way to check if SMT is enabled before starting the VCPU,
and as this case is the only one setting errno to EBUSY, we just check
the errno value to display a message.

Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Message-Id: <1431976007-20503-1-git-send-email-lvivier@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
