xen/arm: p2m: Extend p2m_get_entry to return the value of bit[0] (valid bit)

xen/arm: p2m: Extend p2m_get_entry to return the value of bit[0] (valid bit)

With the recent changes, a P2M entry may be populated but may not be
valid. In some situation, it would be useful to know whether the entry
has been marked available to guest in order to perform a specific
action. So extend p2m_get_entry to return the value of bit[0] (valid bit).

Signed-off-by: Julien Grall <julien.grall@arm.com>
Acked-by: Razvan Cojocaru <rcojocaru@bitdefender.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>