Handle PBA that shares a page with MSI-X table for passthrough devices.

[jhb]

Summary

Handle PBA that shares a page with MSI-X table for passthrough devices.

If the PBA shares a page with the MSI-X table, map the shared page via
/dev/mem and emulate accesses to the portion of the PBA in the shared
page by accessing the mapped page.

Test Plan

• Pass through a device whose PBA shares a page with the MSI-X table to a bhyve guest. Note that the driver for this device doesn't use the PBA, so I haven't actually tested PBA access.

[grehan]

4096 should eventually be PAGE_SIZE but you inherited that :)