x86 hvm: Improve test for wait-for-SIPI state of a target VLAPIC.

x86 hvm: Improve test for wait-for-SIPI state of a target VLAPIC.

This fixes a livelock in hvmloader with credit2 scheduler, whereby an
AP can be brought online, do its work, and shut itself down, before
the BSP re-emulates the VLAPIC write that sent the SIPI. BSP then ends
up in an endless re-emulation work where it sees the target vcpu is
down, therefore schedules a tasklet, which does no work because the
vcpu is already initialised. The fix is to check v->is_initialised
rather than VPF_down, before scheduling the tasklet.

Signed-off-by: Keir Fraser <keir.fraser@citrix.com>