How to best make include/hw/pci/pcie_sriov.h self-contained

[Markus Armbruster <armbru@redhat.com>]

pcie_sriov.h needs PCI_NUM_REGIONS from pci.h, but doesn't include it.
pci.h must be included before pcie_sriov.h or else compile fails.

Adding #include "pci/pci.h" to pcie_sriov would be wrong, because it
would close an inclusion loop: pci.h includes pcie.h (for
PCIExpressDevice) includes pcie_sriov.h (for PCIESriovPF) includes pci.h
(for PCI_NUM_REGIONS).

The obvious solution is to move PCI_NUM_REGIONS pci.h somewhere
pcie_sriov.h can include without creating a loop.

We already have a few headers that don't include anything: pci_ids.h,
pci_regs.h (includes include/standard-headers/linux/pci_regs.h, which
doesn't count), pcie_regs.h.  Moving PCI_NUM_REGIONS to one of these
would work, but it doesn't feel right.

We could create a new one, say pci_defs.h.  Just for PCI_NUM_REGIONS
feels silly.  So, what else should move there?

Any other ideas?

In case you wonder why I bother you with this...

Back in 2016, we discussed[1] rules for headers, and these were
generally liked:

1. Have a carefully curated header that's included everywhere first.  We
   got that already thanks to Peter: osdep.h.

2. Headers should normally include everything they need beyond osdep.h.
   If exceptions are needed for some reason, they must be documented in
   the header.  If all that's needed from a header is typedefs, put
   those into qemu/typedefs.h instead of including the header.

3. Cyclic inclusion is forbidden.

I'm working on patches to get include/ closer to obeying 2.

[1] Message-ID: <87h9g8j57d.fsf@blackfin.pond.sub.org>
    https://lists.nongnu.org/archive/html/qemu-devel/2016-03/msg03345.html