hw/vfio: Move the IGD quirk code to a separate file

hw/vfio: Move the IGD quirk code to a separate file

The IGD quirk code defines a separate device, the so-called
"vfio-pci-igd-lpc-bridge" which shows up as a user-creatable
device in all QEMU binaries that include the vfio code. This
is a little bit unfortunate for two reasons: First, this device
is completely useless in binaries like qemu-system-s390x.
Second we also would like to disable it in downstream RHEL
which currently requires some extra patches there since the
device does not have a proper Kconfig-style switch yet.

So it would be good if the device could be disabled more easily,
thus let's move the code to a separate file instead and introduce
a proper Kconfig switch for it which gets only enabled by default
if we also have CONFIG_PC_PCI enabled.

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