console: simplify screendump

Screendumps are alot simpler as we can update non-active
QemuConsoles now.  So we only need to update the QemuConsole
we want write out, then dump the DisplaySurface content into
a ppm file.  Done.

No console switching needed.  No special support code in the
gfx card emulation needed.  Zap it all.  Also move ppm_save
out of the vga code and next to the qmp_screendump function.

For now screen dumping is limited to console #0 (like it used
to be), even though it is dead simple to extend it to other
consoles.  I wanna finish the console cleanup before setting
new qapi interfaces into stone.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Tested-by: Igor Mitsyanko <i.mitsyanko@gmail.com>
