common: introduce common module

This module's purpose is to provide shared functions, constants, etc.
for storage plugins and storage-related operations.

It starts out with a align_size_up() function, that will (initially)
be used for volume import.

[FE: start out with a different function for my use case
     fixup Makefile]

Originally-by: Max Carrara <m.carrara@proxmox.com>
Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>

src/PVE/Storage/Makefile

@@ -1,4 +1,5 @@
 SOURCES= \
+	Common.pm \
 	Plugin.pm \
 	DirPlugin.pm \
 	LVMPlugin.pm \
@@ -18,5 +19,6 @@ SOURCES= \
 
 .PHONY: install
 install:
+	make -C Common install
 	for i in ${SOURCES}; do install -D -m 0644 $$i ${DESTDIR}${PERLDIR}/PVE/Storage/$$i; done
 	make -C LunCmd install