exec: optimize phys_page_set_level

exec: optimize phys_page_set_level

phys_page_set_level is writing zeroes to a struct that has just been
filled in by phys_map_node_alloc.  Instead, tell phys_map_node_alloc
whether to fill in the page "as a leaf" or "as a non-leaf".

memcpy is faster than struct assignment, which copies each bitfield
individually.  A compiler bug (https://gcc.gnu.org/PR66391), and
small memcpys like this one are special-cased anyway, and optimized
to a register move, so just use the memcpy.

This cuts the cost of phys_page_set_level from 25% to 5% when
booting qboot.

Reviewed-by: Stefan Hajnoczi <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>