When a handle being passed down to VMCIDoorbell_Create() was not initialized

When a handle being passed down to VMCIDoorbell_Create() was not initialized
to VMCI_INVALID_HANDLE, a doorbell was being created with context id 0
(hypervisor id) and a large resource id (likely the garbage value in
handle.resource).

On the guest side, modified VMCINotifyHashAddEntry so that if the handle
passed down is not VMCI_INVALID_HANDLE and if the context id of the handle
is not the context id of the VM, VMCI_ERROR_INVALID_ARGS will be returned.

Signed-off-by: Marcelo Vanzin <mvanzin@vmware.com>