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

Pirqs are not freed when driver unload, then new pirqs are allocated when
driver reload. 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>
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
