exec: change well-known physical sections to macros

exec: change well-known physical sections to macros

Sections like phys_section_unassigned always have fixed address
in phys_sections.  Declared as macro, so we can use them
when having more than one phys_sections array.

Signed-off-by: Liu Ping Fan <pingfank@linux.vnet.ibm.com>
Signed-off-by: Liu Ping Fan <qemulist@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>