xenapi: Added iPXE ISO boot support

This patch adds support for a new kind of Glance image, a specially crafted ISO
which supports iPXE booting, giving customers a means to roll their own image.

Two virt-layer modifications were needed. The first was adding configurations
for the iPXE ISO feature (network to use, boot menu, mkisofs_cmd). The second
was the ability to inject networking info into the ISO after it was
downloaded.

To use this feature, operators should enable the `ipxe_boot` image-property.

DocImpact

Implements blueprint xenapi-ipxe-iso-boot-support

Change-Id: I33acf9dfdff0a5ed9797723a142bc451348e8549
Merge "xenapi: Added iPXE ISO boot support"