dm: add BIOS/ROM image loading support at High BIOS region

Generic infrastructure for loading BIOS/ROM and providing EPT pages at
High BIOS region.

The size of High BIOS is rounded up to a multiple of 2MB.

v2 -> v3:
- refine mmap_hugetlbfs* to reduce code replication

v1 -> v2:
- make this code generic instead of OVMF-specific

Tracked-On: #1832
Signed-off-by: Peter Fang <peter.fang@intel.com>
Acked-by: Anthony Xu <anthony.xu@intel.com>

devicemodel/core/main.c

@@ -237,6 +237,14 @@ virtio_uses_msix(void)
 	return virtio_msix;
 }
 
+size_t
+high_bios_size(void)
+{
+	size_t size = 0;
+
+	return roundup2(size, 2 * MB);
+}
+
 static void *
 start_thread(void *param)
 {