vmnet: stop recieving events when VM is stopped

vmnet: stop recieving events when VM is stopped

When the VM is stopped using the HMP command "stop", soon the handler will
stop reading from the vmnet interface. This causes a flood of
`VMNET_INTERFACE_PACKETS_AVAILABLE` events to arrive and puts the host CPU
at 100%. We fix this by removing the event handler from vmnet when the VM
is no longer in a running state and restore it when we return to a running
state.

Signed-off-by: Joelle van Dyne <j@getutm.app>
Signed-off-by: Jason Wang <jasowang@redhat.com>