error: Add an accessor for progname

We'd like to get the progname for help output, so add an accessor.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Reviewed-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Reviewed-by: Andreas F=E4rber <afaerber@suse.de>
Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>