target/i386/tcg: check for correct busy state before switching to a new task

target/i386/tcg: check for correct busy state before switching to a new task

This step is listed in the Intel manual: "Checks that the new task is available
(call, jump, exception, or interrupt) or busy (IRET return)".

The AMD manual lists the same operation under the "Preventing recursion"
paragraph of "12.3.4 Nesting Tasks", though it is not clear if the processor
checks the busy bit in the IRET case.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>