mdrestore: Add open_device(), read_header() and show_info() functions

mdrestore: Add open_device(), read_header() and show_info() functions

This commit moves functionality associated with opening the target device,
reading metadump header information and printing information about the
metadump into their respective functions. There are no functional changes made
by this commit.

Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: Chandan Babu R <chandan.babu@oracle.com>
Signed-off-by: Carlos Maiolino <cem@kernel.org>