run_tests: put logs into per-test file

We were using test.log before to keep all the test logs. This patch
creates one log file per test case under logs/ directory with name
"TESTNAME.log". Meanwhile, we will keep the last time log into
logs.old/.

Renaming scripts/functions.bash into scripts/common.bash to store some
more global variables.

Signed-off-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>