Add lvm_vg_* APIs to create and modify VGs.

Add lvm_vg_* APIs to create and modify VGs.

Add some liblvm APIs for VGs.  Most of these APIs simply call into the internal
liblvm library.  Ideally we should call the liblvm functions directly from
the tools.  However, until we convert more of the code to liblvm functions,
things like the cmd_context will get in the way.  For now just implement the
liblvm functions as wrappers around the internal functions, with a little
error checking and return code handling.  We put all these vg APIs into a
new file, lvm_vg.c

The following APIs are implemented:
lvm_vg_create, lvm_vg_extend, lvm_vg_set_extent_size, lvm_vg_write,
lvm_vg_remove, lvm_vg_close.

Still TODO:
- cleanup error handling by using lvm_errno() and related APIs
- cleanup naming / clarify which functions commit to disk vs not
- implement more 'set' functions
- decide on 'set' / 'change' nomenclature

Signed-off-by: Dave Wysochanski <dwysocha@redhat.com>
Acked-by: Alasdair G Kergon <agk@redhat.com>