xen/arm64: Correctly compute the virtual address in maddr_to_virt()

xen/arm64: Correctly compute the virtual address in maddr_to_virt()

The helper maddr_to_virt() is used to translate a machine address to a
virtual address. To save some valuable address space, some part of the
machine address may be compressed.

In theory the PDX code is free to compress any bits so there are no
guarantee the machine index computed will be always greater than
xenheap_mfn_start. This would result to return a virtual address that is
not part of the direct map and trigger a crash at least on debug-build later
on because of the check in virt_to_page().

A recently reverted patch (see 1191156361 "xen/arm: fix mask calculation
in pdx_init_mask") allows the PDX to compress more bits and triggered a
crash on AMD Seattle Platform.

Avoid the crash by keeping track of the base PDX for the xenheap and use
it for computing the virtual address.

Note that virt_to_maddr() does not need to have similar modification as
it is using the hardware to translate the virtual address to a machine
address.

Take the opportunity to fix the ASSERT() as the direct map base address
correspond to the start of the RAM (this is not always 0).

Signed-off-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
(cherry picked from commit 612d476e74a314be514ee6a9744eea8db09d32e5)