Share the Bootloader module across multiple backends

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
xenlight: PV NICs should be NIC_TYPE_VIF; HVM NICs should be NIC_TYPE_VIF_IOEMU

It's an error to request a NIC_TYPE_VIF_IOEMU for PV domains.
We want HVM domains always to have an emulated NIC in case the VM has
no PV drivers.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
xenlight: use eliloader to install PV guests

Since eliloader uses a different argument passing convention to
pygrub, we simply run it first and then tell libxl that we have
a kernel and ramdisk in our hand.

Fixes #21

Signed-off-by: David Scott <dave.scott@eu.citrix.com>