vmm: Introduce a SerialBuffer for buffering serial output

Introduce a dynamic buffer for storing output from the serial port. The
SerialBuffer implements std::io::Write and can be used in place of the
direct output for the serial device.

The internals of the buffer is a vector that grows dynamically based on
demand up to a fixed size at which point old data will be overwritten.
Currently the buffer is only flushed upon writes.

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

vmm/src/lib.rs

@@ -59,6 +59,7 @@ pub mod vm;
 
 #[cfg(feature = "acpi")]
 mod acpi;
+mod serial_buffer;
 
 type GuestMemoryMmap = vm_memory::GuestMemoryMmap<AtomicBitmap>;
 type GuestRegionMmap = vm_memory::GuestRegionMmap<AtomicBitmap>;