libvirt.test: Add some more normal/error cases for qemu-ga test

Add test for 'guest-ping', 'guest-fstrim', 'guest-fsfreeze-thaw',
'guest-fsfreeze-freeze', 'guest-fsfreeze-status' command

Signed-off-by: Li Yang <liyang.fnst@cn.fujitsu.com>
libvirt.test: Skip test if option doesn't exist

Signed-off-by: Li Yang <liyang.fnst@cn.fujitsu.com>