New struct Location holds a location.  So far, the only location is
LOC_NONE, so this doesn't do anything useful yet.

Passing the current location all over the place would be too
cumbersome.  Hide it away in static cur_loc instead, and provide
accessors.  Print it in error_report().

Store it in QError, and print it in qerror_print().

Store it in QemuOpt, for use by qemu_opts_foreach().  This makes
error_report() do the right thing when it runs within
qemu_opts_foreach().

We may still have to store it in other data structures holding user
input for better error messages.  Left for another day.