qemu-config: Accept empty option values

Currently it is impossible to set an option in a config file to an empty
string, because the parser matches only lines containing non-empty
strings between double-quotes.

As sscanf() "[" conversion specifier only matches non-empty strings, add
a special case for empty strings.

Reviewed-by: Eric Blake <eblake@redhat.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>