KVM: PPC: Book3S HV: Add guest->host real mode completion counters

Add counters to track number of times we switch from guest real mode
to host virtual mode during an interrupt-related hyper call because the
hypercall requires actions that cannot be completed in real mode. This
will help when making optimizations that reduce guest-host transitions.

It is safe to use an ordinary increment rather than an atomic operation
because there is one ICP per virtual CPU and kvmppc_xics_rm_complete()
only works on the ICP for the current VCPU.

The counters are displayed as part of IPC and ICP state provided by
/sys/debug/kernel/powerpc/kvm* for each VM.

Signed-off-by: Suresh Warrier <warrier@linux.vnet.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Alexander Graf <agraf@suse.de>