vmm: add pci_segment mmio aperture configs

When using multiple PCI segments, the 32-bit and 64-bit mmio
aperture is split equally between each segment. Add an option
to configure the 'weight'. For example, a PCI segment with a
`mmio32_aperture_weight` of 2 will be allocated twice as much
32-bit mmio space as a normal PCI segment.

Signed-off-by: Thomas Barrett <tbarrett@crusoeenergy.com>

fuzz/fuzz_targets/http_api.rs

@@ -187,6 +187,7 @@ impl RequestHandler for StubApiRequestHandler {
                 watchdog: false,
                 #[cfg(feature = "guest_debug")]
                 gdb: false,
+                pci_segments: None,
                 platform: None,
                 tpm: None,
                 preserved_fds: None,