vz: implement memory setting functions

vz: implement memory setting functions

Quite straigthforward as vz sdk memory setting function makes
just what we want to that is set "amount of physical memory
allocated to a domain".

'useflags' is introduced for non flag function implementation.
We can't just use combination of flags like "live | config" or
we fail for inactive domains. Other combinations have drawbacks
too.

Signed-off-by: Nikolay Shirokovskiy <nshirokovskiy@virtuozzo.com>