Create Drive.pm and move drive-related code there

Create Drive.pm and move drive-related code there

The initialization for the drive keys in $confdesc is changed
to be a single for-loop iterating over the keys of $drivedesc_hash and
the initialization of the unusedN keys is move to directly below it.

To avoid the need to change all the call sites, functions with more than
a few callers are exported from the submodule and imported into QemuServer.pm.

For callers of the now imported functions within QemuServer.pm, the prefix
PVE::QemuServer is dropped, because it is unnecessary and now even confusing.

Signed-off-by: Fabian Ebner <f.ebner@proxmox.com>