x86/kvm: Fix pvclock vsyscall fixmap

The physical memory fixmapped for the pvclock clock_gettime vsyscall
was allocated, and thus is not a kernel symbol. __pa() is the proper
method to use in this case.

Fixes the crash below when booting a next-20130204+ smp guest on a
3.8-rc5+ KVM host.

[    0.666410] udevd[97]: starting version 175
[    0.674043] udevd[97]: udevd:[97]: segfault at ffffffffff5fd020
     ip 00007fff069e277f sp 00007fff068c9ef8 error d

Acked-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Gleb Natapov <gleb@redhat.com>