[PCIe] Support PCIe in Firecracker

[bchalios]

This issue tracks the work for adding PCIe support in Firecracker.

Initially, PCI support in Firecracker was considered as a means to add support for device pass-through (VFIO). However, PCI also enables us to switch to a PCI bus for VirtIO devices. virtio-pci provides a more efficient way to signal VirtIO devices comparing to our current MMIO bus.

As part of this issue we will:

1. Add basic PCIe emulation capabilities in Firecracker
2. Add a virtio-pci implementation and allow VirtIO devices to use this for communicating with the guest.

Work related with this issue will NOT add support for VFIO, neither support for PCI hot-plugging capabilities. Considerations regarding these features are discussed with the community here: #4845.