Avoid to open a virtual drive if no content is present

[digant73]

Related to #18648. A fix in case:

• virtual drives (e.g. provided by power iso etc.) are configured as current VFS games folder but no disc is mounted
• an external SCSI HDD is wrongly detected as CDROM instead of removable HDD (thanks to powerful API provided by Windows). The issue is now by-passed in case the configured base folder for games is not the root folder (e.g. D:/rpcs3 where D:/ is a SCSI HDD (wrongly) discovered by Windows as CDROM)

NOTE: With the exception of virtual drives provided by Windows, virtual drives provided by other tools (e.g. power iso) are not supported (they will not work even mounting a valid ISO game).

MINOR IMPROVEMENT:

• Skip a useless check to detect an optical raw device if navigating on subfolders (e.g. C:/subfolder_1/subfolder_2/), it means we are on a hdd/ssd. A path for an optical drive should include only the drive letter (e.g. E:/). This small optimization also allows the fix reported on second bullet above without the need to replace the used GetDriveType() function with a more complex code (e.g. based on DeviceIoControl() function)