i386: split cpu accelerators from cpu.c, using AccelCPUClass

i386: split cpu accelerators from cpu.c, using AccelCPUClass

i386 is the first user of AccelCPUClass, allowing to split
cpu.c into:

cpu.c            cpuid and common x86 cpu functionality
host-cpu.c       host x86 cpu functions and "host" cpu type
kvm/kvm-cpu.c    KVM x86 AccelCPUClass
hvf/hvf-cpu.c    HVF x86 AccelCPUClass
tcg/tcg-cpu.c    TCG x86 AccelCPUClass

Signed-off-by: Claudio Fontana <cfontana@suse.de>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
[claudio]:
Rebased on commit b8184135 ("target/i386: allow modifying TCG phys-addr-bits")

Signed-off-by: Claudio Fontana <cfontana@suse.de>
Message-Id: <20210322132800.7470-5-cfontana@suse.de>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>