In order to capture any qemu crashes, introduce a method
that scans qemu output and throws an error if a core dump
condition is found.

Improve verify_alive() by calling this function, among
others, and add a check on two of the most called functions
of the framework: vm.wait_for_login() and
vm.wait_for_serial_login().
Signed-off-by: NLucas Meneghel Rodrigues <lmr@redhat.com>