GDB: add `--gdb-prerun-commands` option to GDB plugin

This feature allows custom GDB commands to be executed before the
actual binary is run. What it can do then, is only limited by the
user's goals.

This is intended to allow complex applications, such as QEMU,
to be debugged inside Avocado tests. Configuring GDB for QEMU,
for example, involves passing signals to the application that
would otherwise result in an event that would stop the debugger.

Signed-off-by: Cleber Rosa <crosa@redhat.com>