CLOUDSTACK-2554: CloudStack fails to load XCP 1.6 hypervisors

Missing default constructor fails the agent manager reloading the XCP
resource on reboot of management server. This is fixed by using the
default constructor as do other Xen resources and include a new resource
ala XenServers for XCP1.6.

Signed-off-by: Prasanna Santhanam <tsp@apache.org>
Fixing multiple minor annoyances:

1. Keeping the description consistent - Memory not RAM when referring to
overcommit
2. getters And setters grouped, provided right casing.
3. Removed wildcard imports

Signed-off-by: Prasanna Santhanam <tsp@apache.org>
CLOUDSTACK-2554: Ensuring that we honor hypervisor xen's memory constraints

When setting memory constraints on Xen guests we should honor:
static-min <= dynamic-min <= dynamic-max <= static-max

Our VmSpec while allows the guests to like between dynamic-min and
dynamic-max the memory set by the resource set the static min to be
equal to the dynamic max. This restricts the hypervisor from ensuring
optimized memory handling of guests.

see: http://wiki.xen.org/wiki/XCP_FAQ_Dynamic_Memory_Control#How_does_XCP_choose_targets_for_guests_in_dynamic_range_mode.3F

Another fix was related the restrict_dmc option. when enabled (true)
this option disallows scaling a vm. The logic was reverse handled
allowing scaling when restrict_dmc was on. This control flow is now
fixed.

Signed-off-by: Prasanna Santhanam <tsp@apache.org>
CLOUDSTACK-2554: Incorrect compute of minmemory and cpu

23e54bb0 introduced multiple hypervisors support for cpu and memory
overcommit. Here the HypervisorGuru base which determines the min, max
range for the memory for all hypervisors computes the minCpu using the
MemoryOverCommit ratio and minMemory using the CpuOverCommit ratio.

Minor typo/logic issue but massive damage across all HV if enabled ;)

Signed-off-by: Prasanna Santhanam <tsp@apache.org>