i386: fix read/write cr with icount option

Running Windows with icount causes a crash in instruction of write cr.
This patch fixes it.

Reading and writing cr cause an icount read because there are called
cpu_get_apic_tpr and cpu_set_apic_tpr functions. So, there is need
gen_io_start()/gen_io_end() calls.

Signed-off-by: Mihail Abakumov <mikhail.abakumov@ispras.ru>
Message-Id: <ffb376034ff184f2fcbe93d5317d9e76@ispras.ru>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>