kvm: x86: remove unused but set variable

The local variable *gpa_offset* is set but not used afterwards,
which make the compiler issue a warning with option
-Wunused-but-set-variable. Remove it to avoid the warning.

Signed-off-by: Jiang Biao <jiang.biao2@zte.com.cn>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>