New API: is-lv: check if a block device is a logical volume (RHBZ#619793)

This adds a new API, guestfs_is_lv (g, device), which returns true iff
the named device is an LVM2 logical volume.

A sample guestfish session:

><fs> lvs
/dev/vg_f13x64/lv_root
/dev/vg_f13x64/lv_swap
><fs> list-devices
/dev/vda
><fs> list-partitions
/dev/vda1
/dev/vda2
><fs> is-lv /dev/vg_f13x64/lv_root
true
><fs> is-lv /dev/vg_f13x64/lv_swap
true
><fs> is-lv /dev/vda
false
><fs> is-lv /dev/vda1
false
><fs> is-lv /dev/vda2
false