Rework snapshot code, has_feature

Rework snapshot code, has_feature

Drop snapshot_create, snapshot_delete and snapshot_rollback
in favour of PVE::AbstractConfig. Qemu-specific parts are
implemented in __snapshot_XX methods in PVE::QemuConfig.

has_feature is made an implementation of the abstract
has_feature, and thus moves to PVE::QemuConfig.

Note: a new hook method needed to be introduced to be called
before creating a volume snapshot, after creating a volume
snapshot, and after unfreezing the guestfs after creating a
volume snapshot. The base method in PVE::AbstractConfig is a
noop, the implemention in PVE::QemuConfig runs the necessary
Qemu monitor commands.