x86/EFI: improve boot time diagnostics

To aid analysis of eventual errors, print EFI status codes with error
messages where available.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/x86/efi/boot.c
+++ b/xen/arch/x86/efi/boot.c
@@ -344,11 +344,12 @@ static EFI_FILE_HANDLE __init get_parent
         ret = efi_bs->HandleProtocol(loaded_image->DeviceHandle,
                                      &fs_protocol, (void **)&fio);
         if ( EFI_ERROR(ret) )
-            blexit(L"Couldn't obtain the File System Protocol Interface");
+            PrintErrMesg(L"Couldn't obtain the File System Protocol Interface",
+                         ret);
         ret = fio->OpenVolume(fio, &dir_handle);
     } while ( ret == EFI_MEDIA_CHANGED );
     if ( ret != EFI_SUCCESS )
-        blexit(L"OpenVolume failure");
+        PrintErrMesg(L"OpenVolume failure", ret);
 
 #define buffer ((CHAR16 *)keyhandler_scratch)
 #define BUFFERSIZE sizeof(keyhandler_scratch)
@@ -967,8 +968,8 @@ efi_start(EFI_HANDLE ImageHandle, EFI_SY
 
     if ( !EFI_ERROR(efi_bs->LocateProtocol(&shim_lock_guid, NULL,
                     (void **)&shim_lock)) &&
-         shim_lock->Verify(kernel.ptr, kernel.size) != EFI_SUCCESS )
-        blexit(L"Dom0 kernel image could not be verified.");
+         (status = shim_lock->Verify(kernel.ptr, kernel.size)) != EFI_SUCCESS )
+        PrintErrMesg(L"Dom0 kernel image could not be verified", status);
 
     name.s = get_value(&cfg, section.s, "ramdisk");
     if ( name.s )
@@ -1381,6 +1382,8 @@ efi_start(EFI_HANDLE ImageHandle, EFI_SY
 
     status = efi_bs->GetMemoryMap(&efi_memmap_size, NULL, &map_key,
                                   &efi_mdesc_size, &mdesc_ver);
+    if ( EFI_ERROR(status) )
+        PrintErrMesg(L"Cannot size memory map", status);
     mbi.mem_upper -= efi_memmap_size;
     mbi.mem_upper &= -__alignof__(EFI_MEMORY_DESCRIPTOR);
     if ( mbi.mem_upper < xen_phys_start )
@@ -1389,7 +1392,7 @@ efi_start(EFI_HANDLE ImageHandle, EFI_SY
     status = efi_bs->GetMemoryMap(&efi_memmap_size, efi_memmap, &map_key,
                                   &efi_mdesc_size, &mdesc_ver);
     if ( EFI_ERROR(status) )
-        blexit(L"Cannot obtain memory map");
+        PrintErrMesg(L"Cannot obtain memory map", status);
 
     /* Populate E820 table and check trampoline area availability. */
     e = e820map - 1;