libvirt: Use os_command_line when kernel_id is set

Patch Iad53c687053bdb5469baad4331ea55f428c170b0 breaks libvirt with
"-append only allowed with -kernel option" when using images without an
explicit boot kernel set because the -kernel option (kernel_id in nova)
isn't supplied for those image types.

This patch reads the os_command_line property from the image metadata
only when using images with an explicit boot kernel set and ignores it
otherwise.

Closes-bug: 1300861

Change-Id: I06c2d9930e0f36a0d7057b6a0f5c9c591caac43f
Merge "libvirt: Use os_command_line when kernel_id is set"