cpus: extract out TCG-specific code to accel/tcg

cpus: extract out TCG-specific code to accel/tcg

TCG is the first accelerator to register a "CpusAccel" interface
on initialization, providing functions for starting a vcpu,
kicking a vcpu, sychronizing state and getting virtual clock
and ticks.

Signed-off-by: Claudio Fontana <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Tested-by: Philippe Mathieu-Daudé <[email protected]>
[added const]
Signed-off-by: Claudio Fontana <[email protected]>
Reviewed-by: Richard Henderson <[email protected]>
Reviewed-by: Alex Bennée <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>