VT-d/qinval: make local variable used for communication with IOMMU "volatile"

VT-d/qinval: make local variable used for communication with IOMMU "volatile"

Without that there is - afaict - nothing preventing the compiler from
putting the variable into a register for the duration of the wait loop.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Yang Zhang <yang.z.zhang@intel.com>
master commit: ceec46c02074e1b2ade0b13c3c4a2f3942ae698c
master date: 2014-06-20 10:25:33 +0200