New API: lvm-canonical-lv-name: make LV name canonical.

New API: lvm-canonical-lv-name: make LV name canonical.

When logical volume names appear in places like /etc/fstab
files they can have the form "/dev/mapper/foo-bar".  This
function takes such names and makes them canonical.

Note that this operation cannot be performed using the current
API, because 'guestfs_stat' does not work on device names, and
we don't really want to make a 'stat-device' call since that
exposes too much non-useful detail about the appliance.

With this patch you can do this:

><fs> debug ll /dev/mapper
total 8
drwxrwxr-x  2 root root   4096 Oct 25 12:51 .
drwxr-xr-x 16 root root   4096 Oct 25 12:51 ..
crw-------  1 root root 10, 62 Oct 25 12:51 control
lrwxrwxrwx  1 root root      7 Oct 25 12:51 vg_f13x64-lv_root -> ../dm-0
lrwxrwxrwx  1 root root      7 Oct 25 12:51 vg_f13x64-lv_swap -> ../dm-1

><fs> lvm-canonical-lv-name /dev/mapper/vg_f13x64-lv_root
/dev/vg_f13x64/lv_root
><fs> lvm-canonical-lv-name /dev/mapper/vg_f13x64-lv_swap
/dev/vg_f13x64/lv_swap
><fs> lvm-canonical-lv-name /dev/mapper/foo
libguestfs: error: lvm_canonical_lv_name: lvm_canonical_lv_name_stub: /dev/mapper/foo: No such file or directory
><fs> lvm-canonical-lv-name /dev/mapper/control
libguestfs: error: lvm_canonical_lv_name: /dev/mapper/control: not a logical volume
><fs> lvm-canonical-lv-name /dev/vg_f13x64/lv_root
/dev/vg_f13x64/lv_root