Do not assume program header is sorted by VA

Do not assume program header is sorted ascendantly by virtual
address, otherwise loader_result.kernel_end may be wrong.

Signed-off-by: Liu Jiang <gerry@linux.alibaba.com>

Author: jiangliu

coverage_config_x86_64.json

@@ -1,5 +1,5 @@
 {
-  "coverage_score": 74.7,
+  "coverage_score": 74.8,
   "exclude_path": "",
   "crate_features": ""
 }

src/loader/x86_64/elf/mod.rs

@@ -236,10 +236,11 @@ impl KernelLoader for Elf {
                 .read_exact_from(mem_offset, kernel_image, phdr.p_filesz as usize)
                 .map_err(|_| Error::ReadKernelImage)?;
 
-            loader_result.kernel_end = mem_offset
+            let kernel_end = mem_offset
                 .raw_value()
                 .checked_add(phdr.p_memsz as GuestUsize)
                 .ok_or(KernelLoaderError::MemoryOverflow)?;
+            loader_result.kernel_end = std::cmp::max(loader_result.kernel_end, kernel_end);
         }
 
         // elf image has no setup_header which is defined for bzImage