KVM: get_tss_base_addr() should return a gpa_t

If TSS we are switching to resides in high memory task switch will fail
since address will be truncated. Windows2k3 does this sometimes when
running with more then 4G

Cc: stable@kernel.org
Signed-off-by: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>