builder/virtualbox/iso: fix boot command key output repetition

In my colleague's environment, boot_command outputs redundant letters so it fails to progress.
virtualbox-iso builder executes VBoxManage `keyboardputscancode` each 1-byte scancode, it causes redundant key output when each VBoxManage execution has too long interval. This interval depends machine specs which running packer.
This PR makes `keyboardscancode` output scancodes all at once each character in boot_command.