LXC: resolve issues in lxcDomainSetMaxMemory

LXC: resolve issues in lxcDomainSetMaxMemory

This patch changes the setmaxmem function to support the '--live',
'--config', and '--current' flags by revectoring the code through
the setmem function using the VIR_DOMAIN_MEM_MAXIMUM flag. The
setmem code is refactored to handle both cases depending on the flag.

The changed maxmem code for the MEM_MAXIMUM path will not allow
modification to the memory values of an active guest unless the --config
switch is used.

Signed-off-by: Chen Hanxiao <chenhanxiao@cn.fujitsu.com>