virttest.aexpect: Support commands longer than 2MB

Current kernels support unlimited argument length, the only limit is
stack size. The default stack size is usually 8KB which is sufficient
for ~2MB commands. Use ulimit to increase these limits. The multi_disk
in some configurations attacks the 2MB barier and in the future we
might see this comming (unless we switch to -readconfig).

NOTE: Only superuser can increase the hard limit, anyway if the command
      fails the test continues and tries to execute the cmd anyway.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>