f3probe: add a friendly way to enter sizes

This patch allows any number passed as parameters to use a unit.
For example, the parameter --debug-real-size=1G sets
a real size of 1GB.
f3probe: issue friendly message when no access

This patch makes f3probe issue a friendly error message
when the user doesn't have access to the device to be tested.
f3probe: make it more uniform to add reset methods