x86/xen: fix balloon target initialization for PVH dom0

PVH dom0 re-uses logic from PV dom0, in which RAM ranges not assigned to
dom0 are re-used as scratch memory to map foreign and grant pages.  Such
logic relies on reporting those unpopulated ranges as RAM to Linux, and
mark them as reserved.  This way Linux creates the underlying page
structures required for metadata management.

Such approach works fine on PV because the initial balloon target is
calculated using specific Xen data, that doesn't take into account the
memory type changes described above.  However on HVM and PVH the initial
balloon target is calculated using get_num_physpages(), and that function
does take into account the unpopulated RAM regions used as scratch space
for remote domain mappings.

This leads to PVH dom0 having an incorrect initial balloon target, which
causes malfunction (excessive memory freeing) of the balloon driver if the
dom0 memory target is later adjusted from the toolstack.

Fix this by using xen_released_pages to account for any pages that are part
of the memory map, but are already unpopulated when the balloon driver is
initialized.  This accounts for any regions used for scratch remote
mappings.  Note on x86 xen_released_pages definition is moved to
enlighten.c so it's uniformly available for all Xen-enabled builds.

Take the opportunity to unify PV with PVH/HVM guests regarding the usage of
get_num_physpages(), as that avoids having to add different logic for PV vs
PVH in both balloon_add_regions() and arch_xen_unpopulated_init().

Much like a6aa4eb9, the code in this changeset should have been part of
38620fc4.

Fixes: a6aa4eb9 ('xen/x86: add extra pages to unpopulated-alloc if available')
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Cc: stable@vger.kernel.org
Signed-off-by: Juergen Gross <jgross@suse.com>
Message-ID: <20250407082838.65495-1-roger.pau@citrix.com>