oeqa/selftest/efibootpartition: improve test

oeqa/selftest/efibootpartition: improve test

This test was overly complex with a setUp() method for a single test case,
which was marked as a class function for no good reason.

Generalise the test so that it has the possibility of working on more
machines in the future, add a decorator so that it only runs on
qemux86-64, and respect QEMU_USE_KVM to speed up test execution.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>