vhost-user: Call qemu_socketpair() instead of socketpair()

As the close-on-exec flags is not set on the file descriptors returned
by socketpair() at default, the fds will survive across exec' function.

In the case that exec' function get invoked, such as the live-update feature
which is been developing, it will cause fd leaks.

To address this problem, we should call qemu_socketpair() to create an pair of
connected sockets with the close-on-exec flag set.

Signed-off-by: Guoyi Tu <tugy@chinatelecom.cn>
Reviewed-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <7002b12a5fb0a30cd878e14e07da61c36da72913.1661240709.git.tugy@chinatelecom.cn>