added serial ports communication + Testable trait

Cargo.toml

@@ -19,6 +19,7 @@ bootloader = "0.9"
 volatile = "0.2.6"
 spin = "0.5.2"
 x86_64 = "0.14.2"
+uart_16550 = "0.2.0"
 
 [dependencies.lazy_static]
 version = "1.0"
@@ -26,5 +27,13 @@ features = ["spin_no_std"]
 
 # this enables exiting qemu from the guest system
 [package.metadata.bootimage]
-test-args = ["-device", "isa-debug-exit,iobase=0xf4,iosize=0x04"]
+test-args = [
+  "-device",
+  "isa-debug-exit,iobase=0xf4,iosize=0x04",
+  "-serial",
+  "stdio",
+  "-display",
+  "none",
+]
 test-success-exit-code = 33 # (0x10 << 1) | 1 = 33
+test-timeout = "300" # in seconds