KVM: PPC: Book3S HV: Pull out interrupt-reading code into a subroutine

This moves the code in book3s_hv_rmhandlers.S that reads any pending
interrupt from the XICS interrupt controller, and works out whether
it is an IPI for the guest, an IPI for the host, or a device interrupt,
into a new function called kvmppc_read_intr.  Later patches will
need this.

Signed-off-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Alexander Graf <agraf@suse.de>