icount: clean up cpu_can_io at the entry to the block

Most of IO instructions can be executed only at the end of the block in
icount mode. Therefore translator can set cpu_can_io flag when translating
the last instruction.
But when the blocks are chained, then this flag is not reset and may
remain set at the beginning of the next block.
This patch resets the flag at the entry of any translation block,
making I/O operations impossible by default.

Signed-off-by: Pavel Dovgalyuk <Pavel.Dovgaluk@ispras.ru>

--

v2 changes:
 - reset can_do_io at the start of every TB (suggested by Paolo Bonzini)
Message-Id: <156404428943.18669.15747009371169578935.stgit@pasha-Precision-3630-Tower>

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>