KVM: arm64: Introduce pkvm_alloc_private_va_range()

pkvm_hyp_alloc_private_va_range() can be used to reserve private VA ranges
in the pKVM nVHE hypervisor. Allocations are aligned based on the order of
the requested size.

This will be used to implement stack guard pages for pKVM nVHE hypervisor
(in a subsequent patch in the series).

Credits to Quentin Perret <qperret@google.com> for the idea of moving
private VA allocation out of __pkvm_create_private_mapping()

Signed-off-by: Kalesh Singh <kaleshsingh@google.com>
Tested-by: Fuad Tabba <tabba@google.com>
Reviewed-by: Fuad Tabba <tabba@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220420214317.3303360-3-kaleshsingh@google.com