Libvirt: make boot dev a list in GuestConfig

In order to make boot order more flexible based on the block device
mapping passed, we need to customize the domain xml presented to
libvirt, by specifying more than one boot element. For more info see
http://libvirt.org/formatdomain.html#elementsOSBIOS. Currently the boot
element is hard coded to "hd" by the libvirt driver.

This patch makes it possible to specify more than one boot element that
will give us more flexibility.

Related-bug: 1229327
Change-Id: I38abe9edaa8f93469d58ad81a466f68c2c0d24d6
Merge "Libvirt: make boot dev a list in GuestConfig"