drmgr: ensure pci virtio device removal prior to isolation and poweroff

For virtio devices we write to the devices sysfs remove attribute to
trigger kernel cleanup of the device. However, the call is asynchronous
and the cleanup is placed on a work queue. It is possible that qemu can
tear down the device prior to the kernel finishing cleanup causing a
panic.

This patch polls the device waiting for cleanup prior to notifying qemu
to isolate and power down the device.

Signed-off-by: Tyrel Datwyler <tyreld@linux.vnet.ibm.com>