[CORE] vmm_manager: Introduce per-guest request APIs

There would be many situations in device emulation where we would
require to schedule a work/request for Guest because we have to do
it only in sleepable context. Examples of such situations are:
system reboot, system poweroff/shutdown, PCIe device BAR progamming,
etc.

To simplify writing deferred work/request in above described situations
we introduce per-guest request APIs. This will enable us to schedule a
callback function in system workqueue for given Guest with just one
API call. Also, all per-guest request of a Guest will be ignored if a
Guest is destroyed before they are processed.

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