COW device initialized by zeroing the whole first chunk

To be compliant with lvm, instead of just zeroing the first 32 bit of
the COW device, force to zero the whole first chunk.
Also switch to std::vector to store the zeroes, reducing the stack
growth.

Test: libsnapshot_test
Bug: 139202197
Change-Id: I2d98549528a222d6c27bb566c68477b5ec3add20
Signed-off-by: Alessio Balsini <balsini@google.com>