virttest.aexpect: Make it possible to ignore command exception in cmd().

Support ignore_all_errors parameter in cmd()
When run some clean up command in guest. We may do not want to fail the case
for any exception in the command.
So add ignore_all_errors parameter in cmd() to ignore exception.

Signed-off-by: Feng Yang <fyang@redhat.com>
qemu.tests.stop_continue call session.cmd() with ignore_all_errors=True.

We do not want to fail the case for exception in clean up command.

Signed-off-by: Feng Yang <fyang@redhat.com>