vhost-user-test: no set non-blocking for cal fd less than 0.

vhost-user-test: no set non-blocking for cal fd less than 0.

In the scenario where vhost-user sets eventfd to -1,
qemu_chr_fe_get_msgfds retrieves fd as -1. When vhost_user_read
receives, it does not perform blocking operations on the descriptor
with fd=-1, so non-blocking operations should not be performed here
either.This is a normal use case. Calling g_unix_set_fd_nonblocking
at this point will cause the test to interrupt.

When vhost_user_write sets the call fd to -1, it sets the number of
fds to 0, so the fds obtained by qemu_chr_fe_get_msgfds will also
be 0.

Signed-off-by: Yuxue Liu <yuxue.liu@jaguarmicro.com>
Message-Id: <20240411073555.1357-1-yuxue.liu@jaguarmicro.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>