add memory parser

add memory parser

In preparation to add more properties to the memory configuration like
maximum hotpluggable memory and whether virtio-mem devices should be
used.

This also allows to get rid of the cyclic include of PVE::QemuServer
in the memory module.

Signed-off-by: Alexandre Derumier <aderumier@odiso.com>
[FE: also convert new usage in get_derived_property
     remove cyclic include of PVE::QemuServer
     add commit message]
Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>