Mount snapshotted /system in Virtual A/B devices

Mounting /system in Virtual A/B devices may require the creation of the
associated snapshot devices.
This patch performs all the required initializations prior to attempting
the mount of /system.

Bug: 139157327
Test: manual /system partition mount on VAB device during OTA
Depends-on: I7337bdd38d7016d12d3ee42be1c7893b10e9116d
Change-Id: I71a9dfc57e1a1354f1f1edc5d287aca93c0c8924
Signed-off-by: Alessio Balsini <balsini@google.com>