vmm: Add MMIO support

Add (non-default) support for using MMIO for virtio devices. This can be
tested by:

cargo build --no-default-features --features "mmio"

All necessary options will be included injected into the kernel
commandline.

Fixes: #243

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

vmm/Cargo.toml

@@ -8,6 +8,7 @@ edition = "2018"
 default = []
 acpi = ["acpi_tables","devices/acpi", "arch/acpi"]
 pci_support = ["pci", "vfio", "vm-virtio/pci_support"]
+mmio_support = ["vm-virtio/mmio_support"]
 
 [dependencies]
 acpi_tables = { path = "../acpi_tables", optional = true }