LVM-activate: verify vg or lv validity

When LVM underlying devs disappear, eg iSCSI SAN disks suffer network
disconnect, the pvs/vgs/lvs won't show related PVs/VGs/LVs, but dm
devs won't disappear. This makes the lvm_status() doesn't work as
expect.

This patch uses simple & stupid method "read 1 byte from underlaying
dev" to detect abnormal underlying devices.

Signed-off-by: Zhao Heming <heming.zhao@suse.com>