KVM: x86: map/unmap private slots in __x86_set_memory_region

Otherwise, two copies (one of them never populated and thus bogus)
are allocated for the regular and SMM address spaces.  This breaks
SMM with EPT but without unrestricted guest support, because the
SMM copy of the identity page map is all zeros.

By moving the allocation to the caller we also remove the last
vestiges of kernel-allocated memory regions (not accessible anymore
in userspace since commit b74a07be, "KVM: Remove kernel-allocated
memory regions", 2010-06-21); that is a nice bonus.

Reported-by: Alexandre DERUMIER <aderumier@odiso.com>
Cc: stable@vger.kernel.org
Fixes: 9da0e4d5
Reviewed-by: Radim Krčmář <rkrcmar@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>