libxc: Rework extra module initialisation

libxc: Rework extra module initialisation

This patch use xc_dom_alloc_segment() to allocate the memory space for the
ACPI modules and the SMBIOS modules. This is to replace the arbitrary
placement of 1MB (+ extra for MB alignement) after the hvmloader image.

This patch can help if one add extra ACPI table and hvmloader contain
OVMF (OVMF is a 2MB binary), as in that case the extra ACPI table could
easily be loaded past the address 4MB, but hvmloader use a range of
memory from 4MB to 10MB to perform tests and in the process, clears the
memory, before loading the modules.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>