hw/arm/virt: Check that the CPU realize method succeeded

We were passing a NULL error pointer to the object_property_set_bool()
call that realizes the CPU object. This meant that we wouldn't detect
failure, and would plough blindly on to crash later trying to use a
NULL CPU object pointer. Detect errors and fail instead.

In particular, this will be necessary to detect the user error
of using "-cpu host" without "-enable-kvm" once we make the host
CPU type be registered unconditionally rather than only in
kvm_arch_init().

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