virttest: add interface to save lvm object to env file

register lvm object to env file when use lvm storage, and restore it at
postprocess to clean up new volumes when "force_remove_image = yes";
virttest: LVM storage base module

Signed-off-by: Xu Tian <xutian@redhat.com>
virttest: add emulated lvm support

Signed-off-by: Xu Tian <xutian@redhat.com>
virttest: add lvm storage support for qemu tests

Signed-off-by: Xu Tian <xutian@redhat.com>
shared.cfg: add params for supporting lvm partiton

Signed-off-by: Xu Tian <xutian@redhat.com>