selfetests/bpf: Update vmtest.sh defaults

Increase memory to 4G, 8 SMP core with host cpu passthrough. This
make it run faster in parallel mode and more likely to succeed.

Signed-off-by: Yucong Sun <sunyucong@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20211025223345.2136168-2-fallentree@fb.com