xen: support console_switching between Dom0 and DomUs on ARM

xen: support console_switching between Dom0 and DomUs on ARM

Today Ctrl-AAA is used to switch between Xen and Dom0. Extend the
mechanism to allow for switching between Xen, Dom0, and any of the
initial DomU created from Xen alongside Dom0 out of information provided
via device tree.

Rename xen_rx to console_rx to match the new behavior.

Clarify existing comment about "notify the guest", making it clear that
it is only about the hardware domain.

Switching the console input to domUs started from Xen at boot is
#ifdef'ed to 0 in this patch. The code will be enabled when
vpl011_rx_char_xen is introduced. For now it is disabled for
bisectability.

Signed-off-by: Stefano Stabellini <stefanos@xilinx.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
CC: andrew.cooper3@citrix.com
CC: George.Dunlap@eu.citrix.com
CC: ian.jackson@eu.citrix.com
CC: jbeulich@suse.com
CC: konrad.wilk@oracle.com
CC: tim@xen.org
CC: wei.liu2@citrix.com