KVM: PPC: Book3S: Trim top 4 bits of physical address in RTAS code

The in-kernel emulation of RTAS functions needs to read the argument
buffer from guest memory in order to find out what function is being
requested.  The guest supplies the guest physical address of the buffer,
and on a real system the code that reads that buffer would run in guest
real mode.  In guest real mode, the processor ignores the top 4 bits
of the address specified in load and store instructions.  In order to
emulate that behaviour correctly, we need to mask off those bits
before calling kvm_read_guest() or kvm_write_guest().  This adds that
masking.

Signed-off-by: Paul Mackerras <paulus@samba.org>
Acked-by: Scott Wood <scottwood@freescale.com>