vmm: Add fast path for PCI config IO port

Looking up devices on the port I/O bus is time consuming during the
boot at there is an O(lg n) tree lookup and the overhead from taking a
lock on the bus contents.

Avoid this by adding a fast path uses the hardcoded port address and
size and directs PCI config requests directly to the device.

Command line:
target/release/cloud-hypervisor --kernel ~/src/linux/vmlinux --cmdline "root=/dev/vda1 console=ttyS0" --serial tty --console off --disk path=~/workloads/focal-server-cloudimg-amd64-custom-20210609-0.raw --api-socket /tmp/api

PIO exit: 17913
PCI fast path: 17871
Percentage on fast path: 99.8%

perf before:

marvin:~/src/cloud-hypervisor (main *)$ perf report -g | grep resolve
     6.20%     6.20%  vcpu0            cloud-hypervisor    [.] vm_device::bus::Bus::resolve

perf after:

marvin:~/src/cloud-hypervisor (2021-09-17-ioapic-fast-path *)$ perf report -g | grep resolve
     0.08%     0.08%  vcpu0            cloud-hypervisor    [.] vm_device::bus::Bus::resolve

The compromise required to implement this fast path is bringing the
creation of the PciConfigIo device into the DeviceManager::new() so that
it can be used in the VmmOps struct which is created before
DeviceManager::create_devices() is called.

Signed-off-by: Rob Bradford <robert.bradford@intel.com>

pci/src/lib.rs

@@ -42,3 +42,8 @@ impl PciInterruptPin {
         self as u32
     }
 }
+
+#[cfg(target_arch = "x86_64")]
+pub const PCI_CONFIG_IO_PORT: u64 = 0xcf8;
+#[cfg(target_arch = "x86_64")]
+pub const PCI_CONFIG_IO_PORT_SIZE: u64 = 0x8;