Define cross-driver standardized vm_mode values

Currently the XenAPI driver allows for a 'vm_mode' parameter to
be set against an instance to determine whether it is launched
a paravirt or fullvirt domain. This allows the values 'pv', 'hv'
and 'hvm'.

The libvirt driver also needs to be extended to allow a 'vm_mode'
parameter, and to facilitate deployment of heterogeneous Nova
compute farms, it is desirable to have standardization of the
VM mode values across all drivers.

To address this, the nova.compute.vm_mode module is introduced
which defines a set of constants. The constants provide four
possible vm modes:

 - vm_mode.XEN = 'xen' - for Xen 3.0 paravirt ABI
 - vm_mode.HVM = 'hvm' - for native ABI
 - vm_mode.UML = 'uml' - for User Mode Linux paravirt ABI
 - vm_mode.EXE = 'exe' - for container virt executable ABI

The existing 'pv' value from XenAPI is deprecated, because it
is ambiguous - both Xen and UML are paravirt, and other
paravirt hypervisor ABIs also exist

The existing 'hv' value is also deprecated since it duplicates
the other existing 'hvm' value.

The 'vm_mode.get_from_instance' method will extract the
vm_mode value for an instance. It normalizes the value to
all lower case, translates 'hv' and 'pv' into the new
values for back compat, and validates that the value
matches one of the defined constants

The XenAPI and libvirt drivers are then updated to use the
nova.compute.vm_mode constants instead of bare strings.
The test_xenapi.py test case is updated to set 'vm_mode'
to exercise the new codepaths. A new test_vmmode.py
case is also written to fully exercise the logic

NB, previously the libvirt driver would set 'guest.os_type'
to 'linux' to request Xen paravirt. This is a legacy value,
with libvirt preferring the string 'xen'. So although the
new code sets a different value for Xen paravirt os_type,
the functional result is unchanged.

DocImpact
blueprint hypervisor-code-consolidation
Change-Id: I23efc5dc1528b0d8472d752a8a30f55c85310b21
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>

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