[COMMANDS] Make functions internal to commands as static

As-per our coding convention internal (or non-public or non-exported)
functions of any module should be static. Some how we missed this in
some of our commands hence this patch.

Signed-off-by: Anup Patel <anup@brainfault.org>
[LIBS] Skeletal implementation of virtual screen capturing library

We would be implementing virtual screen capturing as a library so that
screen capturing APIs can be shared between different parts of Xvisor.

This patch add skeletal module for virtual screen capturing library
so that we can first implement vscreen command using these stub APIs.

Signed-off-by: Anup Patel <anup@brainfault.org>
[COMMANDS] Add vscreen command for virtual screen capturing

We have new vscreen command for virtual screen capturing using
vscreen library. The vscreen command mostly works fine except
that vscreen library is still incomplete.

Signed-off-by: Anup Patel <anup@brainfault.org>
[ARM] Enable vscreen library and command for applicable ARM hosts

The vscreen library and command depends on vio/vmm_vdisplay, vio/vmm_vinput,
drv/fb, and drv/input hence enable these for applicable ARM hosts.

Signed-off-by: Anup Patel <anup@brainfault.org>