qemu.tests.cfg: use absolute path for saving file cmds

when launching a testcase the 2nd time w/o kill the alive VM,
the origin pwd dir which boot the VM is removed/differ(the name
could be same, but the inode isn't, lsof -p $PID will show),
thus cases use relevant path would fail as the path is not
longer valid. like qmp_command.memsave qmp_command.pmemsave.
Changelog:
use /var/tmp instead of /tmp

Signed-off-by: Xiaoqing Wei <xwei@redhat.com>