KVM: PPC: Book3S HV: Enable hypervisor virtualization interrupts while in guest

KVM: PPC: Book3S HV: Enable hypervisor virtualization interrupts while in guest

The new XIVE interrupt controller on POWER9 can direct external
interrupts to the hypervisor or the guest.  The interrupts directed to
the hypervisor are controlled by an LPCR bit called LPCR_HVICE, and
come in as a "hypervisor virtualization interrupt".  This sets the
LPCR bit so that hypervisor virtualization interrupts can occur while
we are in the guest.  We then also need to cope with exiting the guest
because of a hypervisor virtualization interrupt.

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>