Drivers: hv: Provide arch-neutral implementation of get_vtl()

Drivers: hv: Provide arch-neutral implementation of get_vtl()

To run in the VTL mode, Hyper-V drivers have to know what
VTL the system boots in, and the arm64/hyperv code does not
have the means to compute that.

Refactor the code to hoist the function that detects VTL,
make it arch-neutral to be able to employ it to get the VTL
on arm64.

Signed-off-by: Roman Kisel <romank@linux.microsoft.com>
Reviewed-by: Michael Kelley <mhklinux@outlook.com>
Reviewed-by: Tianyu Lan <tiala@microsoft.com>
Link: https://lore.kernel.org/r/20250428210742.435282-5-romank@linux.microsoft.com
Signed-off-by: Wei Liu <wei.liu@kernel.org>
Message-ID: <20250428210742.435282-5-romank@linux.microsoft.com>