Documentation: Update README and other resources

Outline Jailhouse AMD-V system requirements and nested SVM setup instructions
in the qemu-vm.cell config and README file. As Linux 3.17 is already released,
reflect this in aforementioned files as well.

Also remove "AMD-V support" item from the TODO list and add AMD IOMMU to it.

Signed-off-by: Valentine Sinitsyn <valentine.sinitsyn@gmail.com>
[Jan: raise host-kernel requirement to 3.18 for AMD]
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>