x86: Fold vmx/vtd_root_cell_shrink into vmx/vtd_cell_init

x86: Fold vmx/vtd_root_cell_shrink into vmx/vtd_cell_init

This primarily fixes a regression of 46ab6c2f1e: Due to that reordering,
we were first adding devices to a new cell, then removing them from the
root cell which effectively disabled them in the context table.

Analyzing the content of vmx/vtd_root_cell_shrink, we are better off
folding them into the corresponding cell_init functions. We fix the
ordering issue while doing this.

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