Introduce console write hypercall to let Uboot could directly
output with xen console, this needs CONFIG_VERBOSE_DEBUG
enabled in xen.
Because input is not a must requirement in android VM,
and develop pvconsole needs more efforts, so let's use
this hypercall first.
Signed-off-by: Peng Fan
(cherry picked from commit 8836c3104a1edfe542e0c1cef6690bc9d3d842a0)