Misc error handling fixes in text mode monitor commands

Misc error handling fixes in text mode monitor commands

A couple of places in the text monitor were overwriting the
'ret' variable with a >= 0 value before success was actually
determined. So later error paths would not correctly return
the -1 value. The drive_add code was not checking for errors
like missing command

* src/qemu/qemu_monitor_text.c: Misc error handling fixes