xenapi: Reduce code duplication in vmops

A code fragment that creates kernel and ramdisk appeared in several
locations in vmops. This eliminates that duplication by wrapping it in
a new _create_kernel_and_ramdisk() method.

Fixes bug 1207474

Change-Id: Id317477cb278aa1b6d038516e202aff1bd6129a6
Merge "xenapi: Reduce code duplication in vmops"