KVM: PPC: Book3S HV: Save and restore FSCR in the P9 path

The Facility Status and Control Register is a privileged SPR that
defines the availability of some features in problem state. Since it
can be written by the guest, we must restore it to the previous host
value after guest exit.

This restoration is currently done by taking the value from
current->thread.fscr, which in the P9 path is not enough anymore
because the guest could context switch the QEMU thread, causing the
guest-current value to be saved into the thread struct.

The above situation manifested when running a QEMU linked against a
libc with System Call Vectored support, which causes scv
instructions to be run by QEMU early during the guest boot (during
SLOF), at which point the FSCR is 0 due to guest entry. After a few
scv calls (1 to a couple hundred), the context switching happens and
the QEMU thread runs with the guest value, resulting in a Facility
Unavailable interrupt.

This patch saves and restores the host value of FSCR in the inner
guest entry loop in a way independent of current->thread.fscr. The old
way of doing it is still kept in place because it works for the old
entry path.

Signed-off-by: Fabiano Rosas <farosas@linux.ibm.com>
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>