Update error return and comments for lvm_list_vg_names/uuids.

Update error return and comments for lvm_list_vg_names/uuids.

The two liblvm functions that return a list of vgnames and vguuids use
cmd->mem to allocate the list.  Make it clear to the caller that this
memory will be freed when the LVM handle is freed.

Clean up and clarify the return value of the functions.  In the
case of a memory allocation error, add a couple log_errnos to the internal
code, and make it clear that memory allocation returns a NULL pointer.
If there are no VGs in the system, the list returned is an empty list.

Make a note of the fact that currently we return hidden VG names, how
these can be detected (always start with "#"), and that they should
not be used.

Author: Dave Wysochanski <dwysocha@redhat.com>