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 simply adds a liberal 3 second wait to give the kernel time
to cleanup the pci device before qemu tears it down.

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