core: Introduce and use for_each_cell

Encapsulate the common pattern to iterate over all cells.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
core/driver: Switch to ID-based cell addressing scheme

Return the cell ID on cell creation and request this ID instead of the
cell name for destruction. Will also help to keep future per-cell
hypercalls simple.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
core: Inline cell_find

Only one user remaining, so it's simpler to merge it into its caller.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>