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: Update nc_copy_between_remotes() functions.

bug fix and add new parameters support.

Signed-off-by: Feng Yang <fyang@redhat.com>
virttest.utils_net: Add set_guest_ip_addr() function.

We could use it update guest's ip.

Signed-off-by: Feng Yang <fyang@redhat.com>
virttest.utils_test: Make ping() and get_loss_ratio() works for windows.

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