[IA64] Fix a bug in INIT handler

[IA64] Fix a bug in INIT handler

I found a bug in INIT handler.  This bug sometimes occurs on
the following conditions.
 1. Create a domVTi
 2. Run a user program on domVTi
 3. Push to INIT switch

When this bug occurs, Xen shows error messages.

This bug occurs if a vCPU of domVTi runs on a pCPU where its
INIT interruption have not occurred yet.  It is because
"arch._thread.on_ustack" member in vcpu structure is always
zero and, accordingly, ar.bspstore doesn't switch to Xen RBS
in the MINSTATE_START_SAVE_MIN_PHYS.

This patch adds a checking of ipsr.vm bit into the
MINSTATE_START_SAVE_MIN_PHYS for domVTi.  If ipsr.vm bit is 1,
ar.bspstore is switched to Xen RBS.

Signed-off-by: Masaki Kanno <kanno.masaki@jp.fujitsu.com>