Virtual Short Concept in an Op-Amp
Positive feedback does not  result in stable, linear operation. In a positive feedback amplifier the output adds to the input, resulting in a larger input, resulting in a larger output. In other words, the output accelerates quickly in 1 direction until it can't anymore. In a circuit the limitation is the power supply and the output of the amplifier will be saturated at/near the power supply voltage.

When the output of the amplifier is saturated at/near the power supply voltage, it is not operating as a linear amplifier. The output voltage can no longer be described as GAIN*INPUT. No matter what the input is, the output is the power supply rail voltage.

The reason why you get a virtual short with negative feedback is because negative feedback stabilizes the op amp in linear operation, and the output voltage can be described as GAIN*INPUT. Since the gain is very high, the input is very small. This is the condition of the virtual short.

This is a general explanation, and is overlooking details such as closed-loop vs open-loop gain. Maybe some one else can provide a more detailed answer.