vga: Fix VGA base address

Physical memory base address is 0xffffc00000000000, so VGA base address
should be 0xffffc000000b8000.

Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Takuya ASADA <syuu@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
vga: Handle line feed

Handle line feed in the VGA.  To access termios flag, add a constructor.

Signed-off-by: Takuya ASADA <syuu@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
VGA: Add input_ready() and readch() stubs

To use VGAConsole class, add empty input_ready() and readch().

Signed-off-by: Takuya ASADA <syuu@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
loader: Add '--vga' to switch console

Add --vga on cmdline, to switch vga console.

Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Takuya ASADA <syuu@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
loader: fix spurious error message

When trying to run a nonexistant program, e.g., "run.py -e xyz", sometimes
(especially in a debug build) we would see the spurious message:

    program xyz returned -16384

The bug is simple: when osv::run returns null, meaning the program was not
found, it does not set the "ret" value. Our code checked if ret!=0 before
checking if osv::run actually found the program, which is wrong. Simply
changing the order of the code solves this bug.

Fixes #156.

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>