libxl: Recognise ARM architectures

libxl: Recognise ARM architectures

Only tested on v7 but the v8 equivalent seems pretty obvious.

XEN_CAP_REGEX already accepts more than it should (e.g. x86_64p or x86_32be)
but I have stuck with the existing pattern.

With this I can create a guest from:
  <domain type='xen'>
    <name>libvirt-test</name>
    <uuid>6343998e-9eda-11e3-98f6-77252a7d02f3</uuid>
    <memory>393216</memory>
    <currentMemory>393216</currentMemory>
    <vcpu>1</vcpu>
    <os>
      <type arch='armv7l' machine='xenpv'>linux</type>
      <kernel>/boot/vmlinuz-arm-native</kernel>
      <cmdline>console=hvc0 earlyprintk debug root=/dev/xvda1</cmdline>
    </os>
    <clock offset='utc'/>
    <on_poweroff>destroy</on_poweroff>
    <on_reboot>restart</on_reboot>
    <on_crash>destroy</on_crash>
    <devices>
      <disk type='block' device='disk'>
        <source dev='/dev/marilith-n0/debian-disk'/>
        <target dev='xvda1'/>
      </disk>
      <interface type='bridge'>
        <mac address='8e:a7:8e:3c:f4:f6'/>
        <source bridge='xenbr0'/>
      </interface>
    </devices>
  </domain>

Using virsh create and I can destroy it too.

Currently virsh console fails with:
  Connected to domain libvirt-test
  Escape character is ^]
  error: internal error: cannot find character device <null>

I haven't investigated yet.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: Eric Blake <eblake@redhat.com>