v2v: Add --no-trim option, allowing fstrim to be suppressed.

Mainly useful for testing whether fstrim is responsible for various
unexplained boot failures.  We can also suggest it in the field.
v2v: Add a source 'window' display type.

This maps to SDL/GTK, ie. qemu opening a window on the desktop.
We only use it for -i disk -o qemu to help with debugging.
v2v: -o qemu: Refactor end of line handling.
v2v: -o qemu: Only add a serial console to Linux guests.

Makes no sense for Windows guests.
v2v: -o qemu: Add a display to the shell script.