xen_disk: fix memory leak

xen_disk: fix memory leak

On ioreq_release the full ioreq was memset to 0, losing all the data
and memory allocations inside the QEMUIOVector, which leads to a
memory leak. Create a new function to specifically reset ioreq.

Reported-by: Maik Wessler <maik.wessler@yahoo.com>
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Backport to qemu-xen-traditional.

Signed-off-by: Matthew Daley <mattd@bugfuzz.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
(cherry picked from commit 96b58a44756a8821c108358439b0f2c06e531159)
(cherry picked from commit 2c4d7159d8a3273e63ce22c7e7544c182f6d2ca5)