vabits - subtract from the 'end' address instead of from 'bits'.
vabits - subtract from the 'end' address instead of from 'bits'.

This is a fixup for the commits on
calc base address on AArch64 based on autodetected VA space size

As reported by: Brian Brooks <brian.brooks@arm.com>
Signed-off-by: Gabriel Ganne <54ed6f2b99047a4f6b8431ec375373608f566aee@enea.com>
Change-Id: Id1bd7b7d7e5c188d8547c46134082bd4563b92db

chrisy/vpp,chrisy/vpp,FDio/vpp,chrisy/vpp,FDio/vpp,chrisy/vpp,FDio/vpp,chrisy/vpp,vpp-dev/vpp,chrisy/vpp,vpp-dev/vpp,chrisy/vpp,vpp-dev/vpp,vpp-dev/vpp,vpp-dev/vpp,FDio/vpp,FDio/vpp,chrisy/vpp,FDio/vpp,FDio/vpp,FDio/vpp,vpp-dev/vpp,vpp-dev/vpp