KVM: selftests: Move GUEST_ASSERT_EQ to utils header

Move GUEST_ASSERT_EQ to a common header, kvm_util.h, for other
architectures and tests to use. Also modify __GUEST_ASSERT so it can be
reused to implement GUEST_ASSERT_EQ.

Signed-off-by: Ricardo Koller <ricarkol@google.com>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210611011020.3420067-5-ricarkol@google.com