libxc: allow HVM guest to have modules

libxc: allow HVM guest to have modules

Lift the loading code out of PVH specific branch. Take the chance to
make the debug message more useful.

Now the code needs to handle virt_base being UNSET_ADDR, which it is
for HVM guest.  In case virt_base is not set, it should be treated as
zero.  In case PVH and PV, virt_base is set by the respective loader
by parsing the binary.

IPXE will be loaded as a module of Rombios.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>