maxmem: implement virDomainSetMaxMemory API of the qemu driver

This patch implements the code to support virDomainSetMaxMemory API,
and to support VIR_DOMAIN_MEM_MAXIMUM flag in qemudDomainSetMemoryFlags function.
As a result, we can change the maximum memory size of inactive QEMU guests.

Signed-off-by: Taku Izumi <izumi.taku@jp.fujitsu.com>