CA-290491: do not leaf coalesce VDIs with allow_caching=true
CA-290491: do not leaf coalesce VDIs with allow_caching=true

SMGC's leaf coalescing isn't meant to happen on VDIs with caching
enabled.

It wasn't an issue before CP-18840, when the leaf coalescing was
completely disabled on file-based SR anyway. With CP-18840, we need
to be able more careful about this. This patch disables leaf
coalescing on cache-enabled VDIs and protect their VHD tree height
from being changed by leaf coalesce (to avoid disabling/enabling
cache unintentionally).

Signed-off-by: Zheng Li <e351a3eb690a359d8e02cebc130a21bc4c39d357@citrix.com>

chandrikas/sm,chandrikas/sm,chandrikas/sm,xapi-project/sm,xapi-project/sm,xapi-project/sm