QFile fails to open files from GVfs FUSE in ReadWrite mode

When trying to open a file in R/W mode from GVfs FUSE mount point, QFile will fail with an unsupported operation error. This happens at least with Samba/CIFS mounts over the network when created with Nautilus.

Opening such files with plain <cstdio> works fine.

Attached is a test case that shows QFile failing in ReadWrite mode only. Sample run below:

$ ./qfile-test /run/user/1000/gvfs/smb-share:server=1.2.3.4,share=hifi
Using test file /run/user/1000/gvfs/smb-share:server=1.2.3.4,share=hifi/qfiled7KMke
fopen('w') OK
fopen('r') OK
fopen('rw') OK
QIODevice::WriteOnly OK
QIODevice::ReadOnly OK
Error opening QFile for QIODevice::ReadWrite: Operation not supported (5)

Affected downstream application bug report:
https://github.com/keepassxreboot/keepassxc/issues/803