Passing in virtual files into the supplementary x2sys modules

**Description of the desired feature**

Crossposting from https://github.com/GenericMappingTools/pygmt/pull/546#discussion_r459366392_. The supplementary [X2SYS](https://docs.generic-mapping-tools.org/6.1/modules.html#x2sys) modules doesn't seem to support virtual files from the GMT C API yet, only ASCII, Binary and NetCDF files according to https://docs.generic-mapping-tools.org/6.1/supplements/x2sys/x2sys_init.html#format-definition-files. This is the error message when I try to pass in a virtualfile to [`x2sys_cross`](https://docs.generic-mapping-tools.org/6.1/supplements/x2sys/x2sys_cross.html):

```python-traceback
x2sys_cross [WARNING]: No time column, use dummy times
Not a valid unit: c [meter assumed]
Not a valid unit: c [meter assumed]
x2sys_cross [ERROR]: x2sys_read_file : Cannot find track @GMTAPI@-S-I-D-M-T-N-000000
(null): Error from fclose [@GMTAPI@-S-I-D-M-T-N-000000]
```

The equivalent non-virtualfile MCWE in GMT would be something like this:

```bash
export X2SYS_HOME=.
gmt x2sys_init GMT -Dxyz -F
gmt x2sys_cross @tut_ship.xyz -TGMT -Qi > tmp.txt
```

**Are you willing to help implement and maintain this feature?** Perhaps, if it's not too hard :smile: