Use &error_fatal when initializing crypto on qemu-{img,io,nbd}

In addition to making the code simpler, this will replace the
long error messages:
  cannot initialize crypto: Unable to initialize GNUTLS library: [...]
  cannot initialize crypto: Unable to initialize gcrypt
with shorter messages:
  Unable to initialize GNUTLS library: [...]
  Unable to initialize gcrypt

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
