cpu-exec: allow temporary disabling icount

This patch is required for deterministic replay to generate an exception
by trying executing an instruction without changing icount.
It adds new flag to TB for disabling icount while translating it.

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

Signed-off-by: Pavel Dovgalyuk <pavel.dovgaluk@ispras.ru>
Message-Id: <20150917162359.8676.77011.stgit@PASHA-ISP.def.inno>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>