disk: Allow settling to occur after reading partitions

disk: Allow settling to occur after reading partitions

https://bugzilla.redhat.com/show_bug.cgi?id=1400475

In order to avoid a possible error as a result of kernel interactions
with the partition helper, let's use virWaitForDevices to force things
to settle down before attempting to open and read the partition. This
is related to https://bugzilla.redhat.com/show_bug.cgi?id=1264719.

Although perhaps overkill to have too many places to settle, since
we know that the act of reading the partitions via libvirt_parthelper
will cause udev activity/events - we just need to ensure udev has
been settled before proceding with usage of the device.

Signed-off-by: John Ferlan <jferlan@redhat.com>
ACKed-by: Michal Privoznik <mprivozn@redhat.com>