kvm: support multiple devices for boot order preference

[gedia]

This patch adds multiple device support for the boot_order
setting. Valid values are still disk, cdrom, network and floppy,
but the user can now supply a comma-separated list of those instead
of a single value only.

Additionally, this patch will now use the newer
'-boot order=drives' syntax for the qemu command line,
instead of the deprecated '-boot drives'.

[saschalucas]

Thanks for your contribution. This looks almost reviewable for me (with my limited capabilities).

Without looking in depth I think the extended boot_order syntax should be documented in the man page of gnt-instance, too.

I wonder if we should not take Ganeti downgrades into account. Say this will be included in Ganeti-3.1, there must be a possibility to trim the extended boot_order list back to a single (the first?) value, so that a downgraded Ganeti-3.0 will understand the cluster config. What do @apoikos and @iustin think?

Ah, and please check maximum line length to be <=80. It looks like there are a view longer one.

[rbott]

I agree with @saschalucas here, we need some downgrade code in lib/tools/cfgupgrade.py. AFAIR this means we have to create a new helper method (e.g. DowngradeBootOrder()) and reference it from DowngradeAll(). Also, the gnt-instance documentation needs an update on the boot_order parameter.

Other than that the QA suite runs just fine with the modified code - LGTM :-)