Fix some syscalls in order to pass basic testcases

[greatbridf]

The current implementation use sdx for virtio disks, while the testcases assumes that those disks should have vdx definition. We can change its name according to Linux block device number specs.

Also, we might need to decide the filesystem type if the type is not provided. The most easy way to do this is to use ext4 for default and fail it is not.

[greatbridf]

Syscall umount should be implemented in the same way as well.