vfio-pci: Generalize PCI config mangling

Kernel-side vfio virtualizes all of config space, but some parts are
unique to Qemu.  For instance we may or may not expose the ROM BAR,
Qemu manages MSI/MSIX, and Qemu manages the multi-function bit so that
single function devices can appear as multi-function and vica versa.
Generalize this into a bitmap of Qemu emulated bits.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
