s390x/kvm: Support access register mode for KVM_S390_MEM_OP ioctl

s390x/kvm: Support access register mode for KVM_S390_MEM_OP ioctl

Access register mode is one of the modes that control dynamic address
translation. In this mode the address space is specified by values of
the access registers. The effective address-space-control element is
obtained from the result of the access register translation. See
the "Access-Register Introduction" section of the chapter 5 "Program
Execution" in "Principles of Operations" for more details.

When the CPU is in AR mode, the s390_cpu_virt_mem_rw() function must
know which access register number to use for address translation.
This patch does several things:
- add new parameter 'uint8_t ar' to that function
- decode ar number from intercepted instructions
- pass the ar number to s390_cpu_virt_mem_rw(), which in turn passes it
to the KVM_S390_MEM_OP ioctl.

Signed-off-by: Alexander Yarygin <yarygin@linux.vnet.ibm.com>
Reviewed-by: Thomas Huth <thuth@linux.vnet.ibm.com>
Reviewed-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>