block: Add special error code for wrong format The block drivers need a special error code for "wrong format". From the available error codes EMEDIUMTYPE fits best. It is not available on all platforms, so a definition in qemu-common.h and a specific error report are needed. Signed-off-by: Stefan Weil <sw@weilnetz.de> Reviewed-by: Eric Blake <eblake@redhat.com> Signed-off-by: Kevin Wolf <kwolf@redhat.com>