xen-block: Fix removal of backend instance via xenstore

xen-block: Fix removal of backend instance via xenstore

Whenever a Xen block device is detach via xenstore, the image
associated with it remained open by the backend QEMU and an error is
logged:
    qemu-system-i386: failed to destroy drive: Node xvdz-qcow2 is in use

This happened since object_unparent() doesn't immediately frees the
object and thus keep a reference to the node we are trying to free.
The reference is hold by the "drive" property and the call
xen_block_drive_destroy() fails.

In order to fix that, we call drain_call_rcu() to run the callback
setup by bus_remove_child() via object_unparent().

Fixes: 2d24a6466154 ("device-core: use RCU for list of children of a bus")

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Reviewed-by: Paul Durrant <paul@xen.org>
Message-Id: <20210308143232.83388-1-anthony.perard@citrix.com>