icount: add sleep parameter to the icount option to set icount_sleep mode

The 'sleep' parameter sets the icount_sleep mode, which is enabled by
default. To disable it, add the 'sleep=no' parameter (or 'nosleep') to the
qemu -icount option.

Signed-off-by: Victor CLEMENT <victor.clement@openwide.fr>
Message-Id: <1432912446-9811-3-git-send-email-victor.clement@openwide.fr>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>