cpu: Introduce TCGCpuOperations struct

The TCG-specific CPU methods will be moved to a separate struct,
to make it easier to move accel-specific code outside generic CPU
code in the future.  Start by moving tcg_initialize().

The new CPUClass.tcg_opts field may eventually become a pointer,
but keep it an embedded struct for now, to make code conversion
easier.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
[claudio: move TCGCpuOperations inside include/hw/core/cpu.h]
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20210204163931.7358-2-cfontana@suse.de>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>