boot-qemu.py: Use context manager for pathlib open() calls

New pylint warns:

  boot-qemu.py:413:8: R1732: Consider using 'with' for resource-allocating operations (consider-using-with)
  boot-qemu.py:418:8: R1732: Consider using 'with' for resource-allocating operations (consider-using-with)

We do not expect these calls to fail but the refactoring is simple to silence
the warning.

Signed-off-by: Nathan Chancellor <nathan@kernel.org>

Author: nathanchance

boot-qemu.py

@@ -411,12 +411,14 @@ def get_efi_args(guest_arch):
 
         efi_img_qemu = base_folder.joinpath("images", guest_arch, "efi.img")
         shutil.copyfile(efi_img, efi_img_qemu)
-        efi_img_qemu.open(mode="r+b").truncate(efi_img_size)
+        with efi_img_qemu.open(mode="r+b") as file:
+            file.truncate(efi_img_size)
 
         efi_vars_qemu = base_folder.joinpath("images", guest_arch,
                                              "efivars.img")
         efi_vars_qemu.unlink(missing_ok=True)
-        efi_vars_qemu.open(mode="xb").truncate(efi_img_size)
+        with efi_vars_qemu.open(mode="xb") as file:
+            file.truncate(efi_img_size)
 
     elif guest_arch == "x86_64":
         efi_img_qemu = efi_img  # This is just usable, it is marked read only