qemu-xen-trad: free all the pirqs for msi/msix when driver unloads

qemu-xen-trad: free all the pirqs for msi/msix when driver unloads

Pirqs are not freed when driver unloads, then new pirqs are allocated when
driver reloads. This could exhaust pirqs if do it in a loop.

This patch fixes the bug by freeing pirqs when ENABLE bit is cleared in
msi/msix control reg.

There is also other way of fixing it such as reuse pirqs between driver reload,
but this way is better.
Xen-devel: http://marc.info/?l=xen-devel&m=136800120304275&w=2

Signed-off-by: Zhenzhong Duan <zhenzhong.duan@oracle.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
[ This is the qemu-xen-trad version of qemu-xen-upstream
  1d4fd4f0e2fc5dcae0c60e00cc9af95f52988050 -iwj ]
(cherry picked from commit d0395cc49b2ec6d1723c01f1daf2394b9264ca29)