target/arm: Move definition of 'host' cpu type into cpu.c

Move the definition of the 'host' cpu type into cpu.c, where all the
other CPU types are defined.  We can do this now we've decoupled it
from the KVM-specific host feature probing.  This means we now create
the type unconditionally (assuming we were built with KVM support at
all), but if you try to use it without -enable-kvm this will end
up in the "host cpu probe failed and KVM not enabled" path in
arm_cpu_realizefn(), for an appropriate error message.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 20180308130626.12393-3-peter.maydell@linaro.org