pci: expose intel_graphics_stolen as sysctl

The Intel graphics stolen memory is used by the Intel GOP driver on
boot. When using bhyve with GPU passthrough, it's also used by the guest
GOP driver at guest boot. For that reason, bhyve needs to know the
address and size of this region to inform the guest about this region.
Exposing the variables as sysctl allows bhyve to easily read them.