vhost: fix wrong usage of eventfd_t

vhost: fix wrong usage of eventfd_t

According to eventfd man page:

    typedef uint64_t eventfd_t;

    int eventfd_read(int fd, eventfd_t *value);
    int eventfd_write(int fd, eventfd_t value);

eventfd_t is defined for the second arg(value), but not for fd.

Here I redefine those fd fields to `int' type, which also removes
the redundant (int) cast. And as the man page stated, we should
cast 1 to eventfd_t type for eventfd_write().

Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Acked-by: Huawei Xie <huawei.xie@intel.com>