accel: split the tcg accelerator from accel.c file

there are some types of accelerators in qemu, and all accelerators
have their own file except tcg. tcg accelerator is also defined in
accel.c file. tcg accelerator file will be splited from accel.c and
re-name to tcg-all.c. accel/ directory will be created to include
kvm and tcg related files.

Signed-off-by: Yang Zhong <yang.zhong@intel.com>
Message-Id: <1496383606-18060-2-git-send-email-yang.zhong@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>