qga: Plug unlikely memory leak in guest-set-memory-blocks

transfer_memory_block() leaks an Error object when reading file
/sys/devices/system/memory/memory<INDEX>/state fails with errno other
than ENOENT, and @sys2memblk is false, i.e. when the state file exists
but cannot be read (seems quite unlikely), and this is
guest-set-memory-blocks, not guest-get-memory-blocks.

Plug the leak.

Fixes: bd240fca
Cc: Michael Roth <mdroth@linux.vnet.ibm.com>
Cc: Hailiang Zhang <zhang.zhanghailiang@huawei.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: zhanghailiang <zhang.zhanghailiang@huawei.com>
Message-Id: <20200630090351.1247703-9-armbru@redhat.com>