s390x: add Protected VM support

Add support for Protected Virtual Machine (PVM) tests. For starting a
PVM guest we must be able to generate a PVM image by using the
`genprotimg` tool from the s390-tools collection. This requires the
ability to pass a machine-specific host-key document, so the option
`--host-key-document` is added to the configure script.

Reviewed-by: Janosch Frank <frankja@linux.ibm.com>
Signed-off-by: Marc Hartmayer <mhartmay@linux.ibm.com>
Message-Id: <20200923134758.19354-5-mhartmay@linux.ibm.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
[thuth: Drop the verbose "SKIP ... (no host-key document specified)" output]
Signed-off-by: Thomas Huth <thuth@redhat.com>