add new bootloader xenpvnetboot for pv guest

add new bootloader xenpvnetboot for pv guest

`xenpvnetboot' supports getting boot images from following locations:

 - http://host/path
 - https://host/path
 - ftp://host/path
 - file:///path
 - tftp://host/path
 - nfs:host:/path
 - /path
 - /path/file.iso
 - /path/filesystem.img
 - /dev/sda1
 - nfs+iso:host:/path/file.iso
 - nfs+iso:host:/path/filesystem.img

To use it, make `xenpvnetboot' as bootloader for PV guest::

    bootloader = '/usr/bin/xenpvnetboot'

To get boot images from various locations, set the right bootloader
arguments, e.g.:

    bootloarder_args = ['--location=http://192.168.0.1/fedora/x86_64']
    bootloarder_args = ['--location=ftp://192.168.0.1/fedora/x86_64']
    bootloarder_args = ['--location=file:///fedora/x86_64']
    bootloarder_args = ['--location=tftp://192.168.0.1/fedora/x86_64']
    bootloarder_args = ['--location=/fedora/x86_64']
    bootloarder_args = ['--location=/fedora/Fedora-16-x86_64-DVD.iso']
    bootloarder_args = ['--location=nfs:192.168.0.1:/fedora/x86_64']
    bootloarder_args = ['--location=nfs+iso:192.168.0.1:/fedora/Fedora-16-x86_64-DVD.iso']

You can use `kernel' and `ramdisk' to specify the relative path of
boot kernel and ramdisk. `xenpvnetboot' will join them with the
location to find the boot kernel and ramdisk, e.g.:

    kernel = 'images/pxeboot/vmlinuz'
    ramdisk = 'images/pxeboot/initrd.img'
    bootloarder_args = ['--location=http://192.168.0.1/fedora/x86_64']

    kernel = 'fedora/x86_64/images/pxeboot/vmlinuz'
    ramdisk = 'fedora/x86_64/images/pxeboot/initrd.img'
    bootloarder_args = ['--location=http://192.168.0.1/']

You can also omit the `--location' option and specify the full URL for
`kernel' and `ramdisk' directly, e.g.:

    kernel = 'http://192.168.0.1/fedora/x86_64/images/pxeboot/vmlinuz'
    ramdisk = 'http://192.168.0.1/fedora/x86_64/images/pxeboot/initrd.img'

If only `--location' is specified and `kernel' and `ramdisk' are not
specified, `xenpvnetboot' will search the following places for boot
images from the location::

    ('images/xen/vmlinuz', 'images/xen/initrd.img'), # Fedora <= 10 and OL = 5
    ('boot/i386/vmlinuz-xen', 'boot/i386/initrd-xen'), # openSUSE >= 10.2 and SLES >= 10
    ('boot/x86_64/vmlinuz-xen', 'boot/x86_64/initrd-xen'), # openSUSE >= 10.2 and SLES >= 10
    ('current/images/netboot/xen/vmlinuz', 'current/images/netboot/xen/initrd.gz'), # Debian
    ('images/pxeboot/vmlinuz', 'images/pxeboot/initrd.img'), # Fedora >=10 and OL >= 6
    ('isolinux/vmlinuz', 'isolinux/initrd.img'), # Fedora >= 10 and OL >= 6

`xenpvnetboot' requires python module `urlgrabber'
http://urlgrabber.baseurl.org/.

Signed-off-by: Zhigang Wang <zhigang.x.wang@oracle.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>