Make hexdump only print the # bytes; add pahexdump; make syscall_table visible

hexdump would print too much; fix that.
pahexdump uses a physical address.
For VMs we need to make the syscall_table visible.

Signed-off-by: Ronald G. Minnich <rminnich@google.com>
Merge branch 'master' of github.com:brho/akaros into testing
Refactored postboot kernel testing classes
Completed testing infrastructure for postboot kernel tests.

* Created a way for registering tests that will be ran after boot in kernel mode.
* Added macros for creating tests that follow a standard assertion pattern.
* Converted several existing tests to the follow this new standard.