discover/devmapper: Disable libdm udev sync support

Certain userspace environments that Petitboot is packaged with include
a libdm built with udev sync support which hung with the previous approach.
Relying on udev to properly process dm device creation makes some
assumptions about the flavour of udev available - until that is certain
disable udev sync support and have device-mapper control device creation
itself.

Signed-off-by: Samuel Mendoza-Jonas <sam.mj@au1.ibm.com>