In case we do request_resource() in vmbus_acpi_add() we need to tear it down
to be able to load the driver again. Otherwise the following crash in observed
when hv_vmbus unload/load sequence is performed on a Generation2 instance:

[   38.165701] BUG: unable to handle kernel paging request at ffffffffa00075a0
[   38.166315] IP: [<ffffffff8107dc5f>] __request_resource+0x2f/0x50
[   38.166315] PGD 1f34067 PUD 1f35063 PMD 3f723067 PTE 0
[   38.166315] Oops: 0000 [#1] SMP
[   38.166315] Modules linked in: hv_vmbus(+) [last unloaded: hv_vmbus]
[   38.166315] CPU: 0 PID: 267 Comm: modprobe Not tainted 3.19.0-rc5_bug923184+ #486
[   38.166315] Hardware name: Microsoft Corporation Virtual Machine/Virtual Machine, BIOS Hyper-V UEFI Release v1.0 11/26/2012
[   38.166315] task: ffff88003f401cb0 ti: ffff88003f60c000 task.ti: ffff88003f60c000
[   38.166315] RIP: 0010:[<ffffffff8107dc5f>]  [<ffffffff8107dc5f>] __request_resource+0x2f/0x50
[   38.166315] RSP: 0018:ffff88003f60fb58  EFLAGS: 00010286
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>