block: do not abuse EMEDIUMTYPE Returning "Wrong medium type" for an image that does not have a valid header is a bit weird. Improve the error by mentioning what format was trying to open it. Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Reviewed-by: Fam Zheng <famz@redhat.com> Signed-off-by: Kevin Wolf <kwolf@redhat.com>