dm vdo: add formatting logic and initialization

dm vdo: add formatting logic and initialization

Add the core formatting logic. The initialization path is updated to
read the geometry block (block 0 on the storage device). If the block
is entirely zeroed, the device is treated as unformatted and
vdo_format() is called. Otherwise, the existing geometry is parsed
and the VDO is loaded as before.

The vdo_format() function initializes the volume geometry and super
block, and marks the VDO as needing it's layout saved to disk.

Signed-off-by: Bruce Johnston <bjohnsto@redhat.com>
Reviewed-by: Matthew Sakai <msakai@redhat.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>