hw/xen/xen-pvh: Reduce included headers

hw/xen/xen-pvh: Reduce included headers

Have "hw/xen/xen-pvh-common.h" include the bare minimal set
of headers. Adapt sources to avoid errors when refactoring
unrelated headers such:

    hw/i386/xen/xen-pvh.c: In function ‘xen_pvh_machine_class_init’:
    hw/i386/xen/xen-pvh.c:84:28: error: ‘TARGET_DEFAULT_CPU_TYPE’ undeclared (first use in this function)
       84 |     mc->default_cpu_type = TARGET_DEFAULT_CPU_TYPE;
          |                            ^~~~~~~~~~~~~~~~~~~~~~~
    hw/xen/xen-pvh-common.c: In function ‘xen_pvh_init’:
    hw/xen/xen-pvh-common.c:217:43: error: ‘MiB’ undeclared (first use in this function)
      217 |         if (s->cfg.pci_ecam.size != 256 * MiB) {
          |                                           ^~~
    hw/xen/xen-hvm-common.c:18:6: error: no previous prototype for ‘xen_mr_is_memory’ [-Werror=missing-prototypes]
       18 | bool xen_mr_is_memory(MemoryRegion *mr)
          |      ^~~~~~~~~~~~~~~~

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@amd.com>
Message-Id: <20250218162618.46167-5-philmd@linaro.org>