translate: early exit in tb_flush if there is no tcg

translate: early exit in tb_flush if there is no tcg

tb_flush does all kind of things, which are very tcg specific. As it
is called from some places even for KVM (e.g. gdb server) it is better
to detect these cases and do an early exit.
This also fixes a crash in the gdb server that was triggered by
commit 909eaac9bbc2 ("tb hash: track translated blocks with qht").

Suggested-by: Paolo Bonzini <[email protected]>
Reported-by: Richard Henderson <[email protected]>
Reported-by: Brent Baccala <[email protected]>
Signed-off-by: Christian Borntraeger <[email protected]>
Message-id: 1472148686[email protected]>
Reviewed-by: Peter Maydell <[email protected]>
Signed-off-by: Peter Maydell <[email protected]>