virgl: Add command and flags to initiate debugging on the host (v2)

virgl: Add command and flags to initiate debugging on the host (v2)

On the host VREND_DEBUG=guestallow must be set to let the guest override
the debug flags.

v2: Send flag string instead of flags, this avoids the need to keep
    the flags in sync.
v3: Only request host logging if the host actually understands the command

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Reviewed-by: Erik Faye-Lund <erik.faye-lund@collabora.com>