virtio-rng: do not use g_assert_cmpint

virtio-rng: do not use g_assert_cmpint

g_assert_cmpint is not available on glib 2.12, which is the minimum
version required to build QEMU (we only require 2.16 to run tests,
since that is the first version including GTester).  Do not use it
in hardware models, use a normal assertion instead.

This fixes the buildbot failure for default_x86_64_rhel5.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>