virtiofs: Fix spelling mistakes

Fix some spelling mistakes in comments:
refernce  ==> reference
happnes  ==> happens
threhold  ==> threshold
splitted  ==> split
mached  ==> matched

Signed-off-by: Zheng Yongjun <zhengyongjun3@huawei.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>