Parted 'invalid option' in XenAPI driver

/sbin/parted is throwing an 'invalid option' error when setting the
partition endpoint to -0 (which tells parted to extend the partition
to the end of the disk). This change adds '--' to indicate the
remaining args are not options.

Fixes bug 1247307

Change-Id: I3bd2e25404d002329aa0ea53e9b60b69c90c0adb
Merge "Parted 'invalid option' in XenAPI driver"