accel/kvm: Check ioctl(KVM_SET_USER_MEMORY_REGION) return value

accel/kvm: Check ioctl(KVM_SET_USER_MEMORY_REGION) return value

kvm_vm_ioctl() can fail, check its return value, and log an error
when it failed. This fixes Coverity CID 1412229:

  Unchecked return value (CHECKED_RETURN)

  check_return: Calling kvm_vm_ioctl without checking return value

Reported-by: Coverity (CID 1412229)
Fixes: 235e8982ad3 ("support using KVM_MEM_READONLY flag for regions")
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Message-Id: <20200221163336.2362-1-philmd@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>