Hi,

I've implemented basic serial console support for x86. The user-visible
features this brings are:

1) For QEMU/KVM, a VGA console is no longer required. i.e. you don't need
X11 to rumprun and get console output.

2) Cloud hypervisors using the hw platform will now get console output.

In the spirit of "convention over configuration", there are no flags to
specify which console to use. The default is to use VGA unless the BIOS
indicates that no VGA is present *and* a serial port is available, in which
case the first serial port will be used, at 115200 baud, 8N1.

I've verified that this approach works fine for local QEMU/KVM and Google
Compute Engine. Amazon EC2 users, please test this on a HVM instance...

I have not yet changed rumprun to use serial console by default. The change
is trivial and is just adding "-nographic -vga none" to the qemu command.
Example for tests/hello/hello.bin:

qemu-system-x86_64 -enable-kvm -nographic -vga none -net none -m 64 -kernel 
tests/hello/hello.bin

(use ^Ax to exit after the kernel has halted)

Note that just -nographic by itself is not enough as this does not actually
remove the VGA device from the VM.

Please review, I'd like to change the rumprun qemu|kvm default console to
serial soonish.

Martin

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