Remove `pagetable_l0` parameter in `virt_to_phys`.

[jounathaen]

Currently, virt_to_phys requires the location of the root-pagetable as a parameter. This value can actually be retrieved from the CPUs registers instead, making the interface easier and more flexible. https://github.com/hermit-os/uhyve/blob/e6ffffa81e04522479f374878da435a4fc99e8e1/src/arch/x86_64/mod.rs#L196-L200