target-arm: Fix incorrect check of kvm_vcpu_ioctl return value

kvm_vcpu_ioctl() returns -ETHING on error, not ETHING -- correct
an incorrect check in kvm_arch_init_vcpu(). This would not have
had any significant ill-effects -- we would just have propagated
the less useful ENOENT up to the caller rather than the more
accurate EINVAL in the unlikely case that the kernel didn't
have VFP-D32 support.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
