xen/arm: domain_build: Don't switch to the guest P2M when copying data

xen/arm: domain_build: Don't switch to the guest P2M when copying data

Until recently, kernel/initrd/dtb were loaded using guest VA and
therefore requiring to restore temporarily the P2M. This was reworked
in a series of commits (up to 9292086 "xen/arm: domain_build: Use
copy_to_guest_phys_flush_dcache in dtb_load") to use a guest PA.

This will also help a follow-up patch which will require
p2m_{save,restore}_state to work in pair to workaround an erratum.

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