features/virtualization: minor updates to gic virtualization doc

Minor updates to skip unbinding the PCIe device (smmute) from its driver
as with distro boot there isn't any driver attached to smmute. In
addition mention about the password prompt when launching lkvm with
sudo permissions.

Signed-off-by: Vivek Gautam <vivek.gautam@arm.com>
Change-Id: Ib5e1c7bd295ee1f8811bdca4f9eeef9d673c0033