Make vg->mda_copies persistent in on disk vg metadata.

Make vg->mda_copies persistent in on disk vg metadata.

This patch adds the ability to read/write the vg->mda_copies values
from/to the vg metadata.

If we read the VG metadata and this field does not exist, we set
mda_copies to the default value of 0.  Later in the code, we use
this special '0' value to indicate a disable of metadata balancing.
This should preserve existing LVM behavior and ensure metadata balancing
can be turned off should the need arise.

Signed-off-by: Dave Wysochanski <dwysocha@redhat.com>