multiboot: compilation fix with DEBUG_MULTIBOOT enabled.

This patch fixes the following compilation errors in multiboot.c
when DEBUG_MULTIBOOT is defined.
Use TARGET_FMT_plx instead of %x for target_phys_addr_t.

  CC    i386-softmmu/multiboot.o
cc1: warnings being treated as errors
qemu/hw/multiboot.c: In function 'mb_add_mod':
qemu/hw/multiboot.c:121: error: format '%08x' expects type 'unsigned int', but argument 4 has type 'target_phys_addr_t'
qemu/hw/multiboot.c:121: error: format '%08x' expects type 'unsigned int', but argument 5 has type 'target_phys_addr_t'
qemu/hw/multiboot.c: In function 'load_multiboot':
qemu/hw/multiboot.c:279: error: format '%#x' expects type 'unsigned int', but argument 5 has type 'target_phys_addr_t'
qemu/hw/multiboot.c:307: error: format '%x' expects type 'unsigned int', but argument 3 has type 'target_phys_addr_t'
qemu/hw/multiboot.c:308: error: format '%x' expects type 'unsigned int', but argument 3 has type 'target_phys_addr_t'
make[1]: *** [multiboot.o] Error 1

Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>