xen-block: stop leaking memory in xen_block_drive_create()

xen-block: stop leaking memory in xen_block_drive_create()

The locally allocated QDict-s need to be freed. ('file_layer' will be
freed implicitly since it is added as an object to 'driver_layer').

Spotted by Coverity: CID 1398649

While in the neighbourhood free 'driver' and 'filename' as soon as they are
added to the QDicts. Freeing after the 'done' label doesn't make that much
sense as, if the error path jumps to that label, the values would be NULL
anyway.

This patch also makes that more obvious by taking the error path if
'params' is NULL and then asserting that both driver and filename are
non-NULL in the normal path.

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Message-Id: <20190219163440.15702-1-paul.durrant@citrix.com>
Acked-by: Anthony PERARD <anthony.perard@citrix.com>
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>