KVM: s390: pv: module parameter to fence asynchronous destroy

Add the module parameter "async_destroy", to allow the asynchronous
destroy mechanism to be switched off. This might be useful for
debugging purposes.

The parameter is enabled by default since the feature is opt-in anyway.

Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Reviewed-by: Janosch Frank <frankja@linux.ibm.com>
Reviewed-by: Steffen Eiden <seiden@linux.ibm.com>
Reviewed-by: Nico Boehr <nrb@linux.ibm.com>
Link: https://lore.kernel.org/r/20221111170632.77622-7-imbrenda@linux.ibm.com
Message-Id: <20221111170632.77622-7-imbrenda@linux.ibm.com>
Signed-off-by: Janosch Frank <frankja@linux.ibm.com>