spapr/xive: add hcall support when under KVM

spapr/xive: add hcall support when under KVM

XIVE hcalls are all redirected to QEMU as none are on a fast path.
When necessary, QEMU invokes KVM through specific ioctls to perform
host operations. QEMU should have done the necessary checks before
calling KVM and, in case of failure, H_HARDWARE is simply returned.

H_INT_ESB is a special case that could have been handled under KVM
but the impact on performance was low when under QEMU. Here are some
figures :

    kernel irqchip      OFF          ON
    H_INT_ESB                    KVM   QEMU

    rtl8139 (LSI )      1.19     1.24  1.23  Gbits/sec
    virtio             31.80    42.30   --   Gbits/sec

Signed-off-by: Cédric Le Goater <[email protected]>
Reviewed-by: David Gibson <[email protected]>
Message-Id: <20190513084245[email protected]>
Signed-off-by: David Gibson <[email protected]>