qemu.tests: enable reboot_time and boot time for Ubuntu

Add modified single_user_cmd and restore_level_cmd for Ubuntu.

Signed-off-by: Chris J Arges <chris.j.arges@canonical.com>
qemu.tests: enable valgrind_memalign for Ubuntu

Use apt-get if guest is Ubuntu.

Signed-off-by: Chris J Arges <chris.j.arges@canonical.com>