docs: update hotplug.md
Currently cloud-hypervisor supports hotplug of CPUs, passthrough devices, virtio devices and memory resizing,
but in hotplug.md it still mentiones: "Currently Cloud Hypervisor only supports hot plugging of CPU devices."
We need to remove the incorrect information from hotplug.md to reflect the true situation.
Fixes:
# Cloud Hypervisor Hot Plug
-Currently Cloud Hypervisor only supports hot plugging of CPU devices.
+Currently Cloud Hypervisor supports hot plugging of CPUs devices (x86 only), PCI devices and memory resizing.
## Kernel support
