vm: Add kva_alloc_aligned

Add a function like kva_alloc that allows us to specify the alignment
of the virtual address space returned.

Reviewed by: alc, kib, markj
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D42788

(cherry picked from commit 839999e7efdc980d5ada92ea93719c7e29765809)