vdpasim: Fix some coccinelle warnings

Fix below warnings reported by coccicheck:

drivers/vdpa/vdpa_sim/vdpa_sim.c:104:1-10: WARNING: Assignment of 0/1 to bool variable
drivers/vdpa/vdpa_sim/vdpa_sim.c:164:7-11: WARNING: Unsigned expression compared with zero: read <= 0
drivers/vdpa/vdpa_sim/vdpa_sim.c:169:7-12: WARNING: Unsigned expression compared with zero: write <= 0

1. The 'ready' variable in vdpasim_virtqueue struct is bool type.
   It is better to initialize vq->ready to false

2. Modify 'read' and 'write' variables type from size_t to ssize_t.
   And preserve the reverse christmas tree ordering of local variables.

Fixes: 2c53d0f6

 ("vdpasim: vDPA device simulator")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Samuel Zou <zou_wei@huawei.com>
Link: https://lore.kernel.org/r/1588990802-28451-1-git-send-email-zou_wei@huawei.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>