virttest.qemu_vm: add support set VGA vram ram vgamem

For qxl device, we can set global default by params like,
qxl_vram_size=67108864 and qxl_vgamem_mb=16. For 'std' VGA
set vgamem_mb like vga_vgamem_mb=16, 'cirrus' VGA set like,
cirrus-vga_vgamem_mb =16.

Signed-off-by: Xu Tian <xutian@redhat.com>
Documentation: Add a CartesianConfigTricks documentation

This is useful for people wanting to execute tests on
a different order than the one they appear in the cartesian
config file. This could eventually grow to add more tips
and tricks involving the cartesian config.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
Merge pull request #1927 from autotest/cartesian_config_tricks_docs

Documentation: Add a CartesianConfigTricks documentation
Merge pull request #1920 from xutian/fix_qxl

virttest.qemu_vm: add support set VGA vram ram vgamem
virttest.utils_net: Add network and ip address helper classes

1) Add a helper class LibvirtNetwort to create a transient network
   for testing.
2) Add a helper class IPAddress for ease manipulating an IPv4 or IPv6
   address.
3) Add a helper function to retrieve all IP addresses from all
   interfaces.

Signed-off-by: Hao Liu <hliu@redhat.com>