x86/boot: introduce boot module types

x86/boot: introduce boot module types

This commit introduces module types for the types of boot modules that may be
passed to Xen. These include xen, kernel, ramdisk, microcode, and xsm policy.
This reduces the need for hard coded order assumptions and global variables to
be used by consumers of boot modules, such as domain construction.

Signed-off-by: Daniel P. Smith <dpsmith@apertussolutions.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>