conf: Introduce dynamicMemslots attribute for virtio-mem

conf: Introduce dynamicMemslots attribute for virtio-mem

Introduced in v8.2.0-rc0~74^2~2, QEMU now allows setting
.dynamic-memslots attribute for virtio-mem-pci devices. When
turned on, it allows memory exposed to guest to be split into
multiple memslots and thus smaller memory footprint (see the
original commit for detailed explanation).

Therefore, introduce new <target/> attribute which will control
that QEMU knob.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>