exec.c: subpage->sub_section is already initialized to 0

In subpage_init(), we will set subpage->sub_section to
PHYS_SECTION_UNASSIGNED by subpage_register. Since
PHYS_SECTION_UNASSIGNED is defined to be 0, and we allocate subpage with
g_malloc0, this means subpage->sub_section is already initialized to 0.

This patch removes the redundant setup for a new subpage and also fix
the code style.

Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
Message-Id: <20190321082555.21118-5-richardw.yang@linux.intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>