Drivers: hv: vmbus: Introduce hv_get_vmbus_root_device()

The ARM64 PCI code for hyperv needs to know the VMBus root
device, and it is private.

Provide a function that returns it. Rename it from "hv_dev"
as "hv_dev" as a symbol is very overloaded. No functional
changes.

Signed-off-by: Roman Kisel <romank@linux.microsoft.com>
Reviewed-by: Michael Kelley <mhklinux@outlook.com>
Link: https://lore.kernel.org/r/20250428210742.435282-10-romank@linux.microsoft.com
Signed-off-by: Wei Liu <wei.liu@kernel.org>
Message-ID: <20250428210742.435282-10-romank@linux.microsoft.com>

include/linux/hyperv.h

@@ -1283,6 +1283,8 @@ static inline void *hv_get_drvdata(struct hv_device *dev)
 	return dev_get_drvdata(&dev->device);
 }
 
+struct device *hv_get_vmbus_root_device(void);
+
 struct hv_ring_buffer_debug_info {
 	u32 current_interrupt_mask;
 	u32 current_read_index;