x86/vmx: Shorten vmx_{get,set}_segment_register() for user segments

x86/vmx: Shorten vmx_{get,set}_segment_register() for user segments

The x86_segment enumeration matches hardware SReg encoding, which can be used
to calculate the appropriate VMCS fields, rather than open coding every
instance.

This reduces the size of the switch statement, and the number of embedded BUG
frames from the __vm{read,write}() calls.  In the unlikely case that a call
does fault, the field can unambiguously be retrieved from the GPR state
printed.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Kevin Tian <kevin.tian@intel.com>