Ensure that shadow_memory is large enough for stable execution

[psrok1]

Very common problem is too low shadow memory limit that results in random crash of Xen VM (output from xl dmesg):

(XEN) d99v0 failed to set entry for 12119 -> 18f17f1 altp2m 0, rc -12
(XEN) domain_crash called from p2m.c:2252
(XEN) Domain 99 (vcpu#0) crashed on cpu#16:
(XEN) ----[ Xen-4.16.0  x86_64  debug=n  Not tainted ]----

The reason is -12 (ENOMEM) caused by too low shadow_memory value set in /etc/drakrun/scripts/cfg.template

Xen recommends By default this is 1MB per guest vCPU plus 8KB per MB of guest RAM. and I think that even the current default of 32MB (recommended by Drakvuf) may be too low for some setups (although I don't have any good tool to measure it). I think we should make a proper note in documentation or check in draksetup to adjust this value.