lib: s390x: Enable reusability of VMs that were in PV mode

Convert the sblk to non-PV when the PV guest is destroyed.

Early return in uv_init() instead of running into the assert. This is
necessary since snippet_pv_init() will always call uv_init().

Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Message-Id: <20221021063902.10878-6-frankja@linux.ibm.com>
Reviewed-by: Steffen Eiden <seiden@linux.ibm.com>
Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>