selftests/bpf: Update vmtests.sh to support aarch64

Add handling of aarch64 when setting QEMU options and provide the right
path to aarch64 kernel image.

Signed-off-by: Manu Bretelle <chantr4@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20221021210701.728135-4-chantr4@gmail.com