devices, vmm: Add legacy CMOS device

Based off of crosvm revision b5237bbcf074eb30cf368a138c0835081e747d71
add a CMOS device. This environments that can't use KVM clock to get the
current time (e.g. Windows and EFI.)

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

vmm/Cargo.toml

@@ -9,6 +9,7 @@ default = []
 acpi = ["acpi_tables","devices/acpi", "arch/acpi"]
 pci_support = ["pci", "vfio", "vm-virtio/pci_support"]
 mmio_support = ["vm-virtio/mmio_support"]
+cmos = ["devices/cmos"]
 
 [dependencies]
 acpi_tables = { path = "../acpi_tables", optional = true }