[CORE] vmm_devdrv: Add public flag in struct vmm_device to disable autoprobe

Currently, autoprobing of newly registered devices on vmm_bus is always
enabled.

We now have situations (such as device pass-through and USB device driver
frameworks) where we need to have autoprobing disabled for certain devices.

This patch adds public flag "autoprobe_disabled" in struct vmm_device
for this purpose.

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