partx: fix --update ranges and out of order tables

partx --update DEVICE NUMBER
was broken in 2 cases:
 * if NUMBER != 1
 * if the partition table was "out of order".
   Ie, where sda2 came after sda3.

References: https://bugs.launchpad.net/ubuntu/+source/cloud-utils/+bug/1244662
Signed-off-by: Scott Moser <smoser@ubuntu.com>
Signed-off-by: Karel Zak <kzak@redhat.com>
tests: update partx sgi test

Since commit 4253275490926cf533bfd0bc33279729e9ff427a the libblkid
lists system partitions for SGI label.

Signed-off-by: Karel Zak <kzak@redhat.com>
tests: add partx --update test

Signed-off-by: Karel Zak <kzak@redhat.com>