[CORE] vio: Add vmm_keymaps APIs

This patch adapts QEMU keymap APIs so that we can now convert
key press events from VNC or GUI daemons into intermediate scancodes.

Further, these intermediate scancodes will be used in keyboard and
mouse emulators for emulating keyboard/mouse events for Guest.

Signed-off-by: Anup Patel <anup@brainfault.org>