KVM guest: fix bogus wallclock physical address calculation

The use of __pa() to calculate the address of a C-visible symbol
is wrong, and can lead to unpredictable results. See arch/x86/include/asm/page.h
for details.

It should be replaced with __pa_symbol(), that does the correct math here,
by taking relocations into account.  This ensures the correct wallclock data
structure physical address is passed to the hypervisor.

Cc: stable@kernel.org
Signed-off-by: Glauber Costa <glommer@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>