[libvirt] libxl: expose and control xen device model versions

[David Scott]

Hi,

These patches expose multiple xen device model options in the
capabilities XML and allow the user to select a specific device
model via the domain XML's <emulator> tag.

It is important to control the device model per VM, since the
default is changing from xen-4.2 "qemu traditional" to xen-4.3
"qemu upstream".

In this proposal, the capabilities XML now has multiple <domain>
elements per <arch> as follows:

    <arch name='i686'>
      <wordsize>32</wordsize>
      <machine>xenfv</machine>
      <domain type='xen'>
        <emulator>/usr/lib64/xen/bin/qemu-dm</emulator>
        <loader>/usr/lib/xen/boot/hvmloader</loader>
      </domain>
      <domain type='xen'>
        <emulator>/usr/lib64/xen/bin/qemu-system-i386</emulator>
        <loader>/usr/lib/xen/boot/hvmloader</loader>
      </domain>
    </arch>

-- is it valid/ sensible to have multiple <domain>'s with the same
'type'? Let me know if you can think of a better mapping.

Comments, criticism welcome.

Cheers,
Dave