virtio-9p: fix build on !CONFIG_UTIMENSAT

virtio-9p: fix build on !CONFIG_UTIMENSAT

This patch introduce a fallback mechanism for old systems that do not
support utimensat().  This fix build failure with following warnings:

hw/virtio-9p-local.c: In function 'local_utimensat':
hw/virtio-9p-local.c:479: warning: implicit declaration of function 'utimensat'
hw/virtio-9p-local.c:479: warning: nested extern declaration of 'utimensat'

and:

hw/virtio-9p.c: In function 'v9fs_setattr_post_chmod':
hw/virtio-9p.c:1410: error: 'UTIME_NOW' undeclared (first use in this function)
hw/virtio-9p.c:1410: error: (Each undeclared identifier is reported only once
hw/virtio-9p.c:1410: error: for each function it appears in.)
hw/virtio-9p.c:1413: error: 'UTIME_OMIT' undeclared (first use in this function)
hw/virtio-9p.c: In function 'v9fs_wstat_post_chmod':
hw/virtio-9p.c:2905: error: 'UTIME_OMIT' undeclared (first use in this function)

[NOTE: At this time virtio-9p is only user of utimensat(), and is available
       only when host is linux and CONFIG_VIRTFS is defined.  So there are
       no similar warning for win32.  Please provide a wrapper for win32 in
       oslib-win32.c if new user really requires it.]

v5:
  - Allow fallback on runtime
  - Move qemu_utimensat() to oslib-posix.c
  - Rebased on latest qemu.git
v4:
  - Use tv_now.tv_usec
v3:
  - Use better alternative handling for UTIME_NOW/OMIT
  - Move qemu_utimensat() to cutils.c
V2:
  - Introduce qemu_utimensat()

Acked-by: Chris Wright <chrisw@sous-sol.org>
Acked-by: M. Mohan Kumar <mohan@in.ibm.com>
Acked-by: Jes Sorensen <Jes.Sorensen@redhat.com>
Signed-off-by: Hidetoshi Seto <seto.hidetoshi@jp.fujitsu.com>
Signed-off-by: Venkateswararao Jujjuri <jvrao@linux.vnet.ibm.com>