Allow raw_read_mda_header to be called from text_label.c.

Allow raw_read_mda_header to be called from text_label.c.

We'd like to pass in mda_header to vgname_from_mda().  In order to
do this, we need to call raw_read_mda_header() from text_label.c,
_text_read(), which gets called from the label_read() path, and
peers into the metadata and update vginfo cache.  We should check
the disable bit here, and if set, not peer into the vg metadata,
thus reducing the I/O to disk.

In the process, move vgname_from_mda() to layout.h, since the fn
only gets called from format_text code, and we need the mda_header
definition from the private layout.h.

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