x86/PVH: correct PVH Dom0 E820 region merge

Folding with an immediately successive region can't be done by merely
lowering the start of the region. The size of the region needs increasing
as well.

Fixes: 5427134eae ("x86: populate PVHv2 Dom0 physical memory map")
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Jason Andryuk <jason.andryuk@amd.com>
Reviewed-by: Roger Pau Monné <roger@xenproject.org>

xen/arch/x86/hvm/dom0_build.c

@@ -229,6 +229,7 @@ static int __init pvh_add_mem_range(struct domain *d, uint64_t s, uint64_t e,
         if ( rs == e && d->arch.e820[i].type == type )
         {
             d->arch.e820[i].addr = s;
+            d->arch.e820[i].size += e - s;
             return 0;
         }