Move ImageTooLarge check to Compute API

The existing ImageTooLarge check is in the compute manager, meaning that a
validation error puts the instance into an ERROR state without the ability to
communicate the fault back to the user.

This patch moves the check to the API where we can return the appropriate
error to the user.

Change-Id: I2183449351e3b70f98d0552ea247184a3641d85a
Remove ImageTooLarge exception

InstanceTypeDiskTooSmall and ImageTooLarge are really two sides of the same
coin; an image is larger than the root_gb of the instance_type. Rather than
have two exceptions for this, this patch consolidates down to just
InstanceTypeDiskTooSmall.

Change-Id: I401205072c111a960beb2932c6c1889141ae03c3
xenapi: Disable VDI size check when root_gb=0

Nova uses root_gb=0 to indicate that we should use the image as-is without
trying to resize the root disk. Currently the xenapi virt-driver doesn't
properly implement this behavior.

This patch gets us a step closer by disabling the VDI size check when
root_gb=0. We still need to ensure that the disk-resize code handle root_gb=0
as well.

References bug 1155113

Change-Id: If91963e079e197a1315c48b8643f3419b42b0b2c
xenapi: Moving tests under tests/virt/xenapi/

We have both a tests/xenapi/ and a tests/virt/xenapi/ directory which
is confusing. This patch standardizes on tests/virt/xenapi/ which
matches the nova/ directory layout.

Change-Id: I324a4730e2477aa1863b17c7ea404ce2932a681c
Merge "Move ImageTooLarge check to Compute API"
Merge "Remove ImageTooLarge exception"
Merge "xenapi: Disable VDI size check when root_gb=0"
Merge "xenapi: Moving tests under tests/virt/xenapi/"
Merge remote-tracking branch 'm_src_repo/citrix-fixes' into build-temp