run_net_test.sh: Add support for arm64

Make the accommodations necessary for arm64 support:

- The compressed kernel image is called something different;
- Switched from isa-serial to pci-serial, as the ARM virt model has no
  support for the PC ISA bus, and it doesn't matter for x86_64;
- Enabled hardware accelerated virtualization only on x86_64.

To test, make sure you have the Android clang and gcc toolchains pathed
for AArch64. Then, run something like the following:

$ CROSS_COMPILE=aarch64-linux-androidkernel- ARCH=arm64 \
    DEFCONFIG=cuttlefish_defconfig ROOTFS=net_test.rootfs.arm64.20181025 \
    /path/to/run_net_test.sh all_tests.sh

All of the changes are keyed off of ARCH=arm64, to minimize the impact
this change has on ARCH=um or ARCH=x86_64.

Bug: 118446450
Change-Id: Id52c4eb061d6da4d3ec3485cd9ca62ff8ef028c2
Signed-off-by: Alistair Strachan <astrachan@google.com>