KVM: s390: Split up __vcpu_run into three parts

In preparation for the following patch (which will change the indentation
of __vcpu_run quite a bit), this patch puts most of the code from __vcpu_run
into separate functions. The first function handles the code that runs
before the SIE instruction and the other one handles the code that runs
afterwards.

Signed-off-by: Thomas Huth <thuth@linux.vnet.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>