Acceptance Test: introduce base class for Linux based tests This introduces a base class for tests that need to interact with a Linux guest. It generalizes the "boot_linux.py" code, already been used by the "virtiofs_submounts.py" and also SSH related code being used by that and "linux_ssh_mips_malta.py". While at it, a number of fixes on hopeful improvements to those tests were added. Changes from v1: * Majority of v1 patches have been merged. * New patches: - Acceptance Tests: make username/password configurable - Acceptance Tests: set up SSH connection by default after boot for LinuxTest - tests/acceptance/virtiofs_submounts.py: remove launch_vm() * Allowed for the configuration of the network device type (defaulting to virtio-net) [Phil] * Fix module name typo (s/qemu.util/qemu.utils/) in the commit message [John] * Tests based on LinuxTest will have the SSH connection already prepared