qemu-xen: use dynticks instead of a static 10ms timeout

qemu-xen: use dynticks instead of a static 10ms timeout

Use dynticks instead of polling the timers every 10ms.
This allows a qemu running in dom0 to wake up only when the next timer
goes off (that is every 100ms, because of the buffer_io_timer) instead
of every 10ms.

For the moment stubdoms still run with the old 10ms timeout because
minios doesn't support the posix timer_create interface yet.

Also disable the nographic_timer when CONFIG_DM because it is only
useful with tcg.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>