selftest: efibootpartition.py: fix QEMU_USE_KVM usage

* if you don't have QEMU_USE_KVM in local.conf it fails with:

  2023-03-12 18:06:29,591 - oe-selftest - DEBUG - Checking if qemux86-64 is not this MACHINE
  2023-03-12 18:06:29,594 - oe-selftest - INFO -  ... ERROR
  2023-03-12 18:06:29,594 - oe-selftest - INFO - Traceback (most recent call last):
    File "/OE/build/poky/meta/lib/oeqa/core/decorator/__init__.py", line 35, in wrapped_f
      return func(*args, **kwargs)
             ^^^^^^^^^^^^^^^^^^^^^
    File "/OE/build/poky/meta/lib/oeqa/selftest/cases/efibootpartition.py", line 18, in test_boot_efi
      if oe.types.qemu_use_kvm(self.td['QEMU_USE_KVM'], self.td["TARGET_ARCH"]):
                               ~~~~~~~^^^^^^^^^^^^^^^^
  KeyError: 'QEMU_USE_KVM'

[YOCTO #12937]

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>