x86/traps: improvements to {rd,wr}msr_hypervisor_regs()

x86/traps: improvements to {rd,wr}msr_hypervisor_regs()

Coverity ID: 1055249 1055250

Coverity was complaining that the switch statments contained dead code in
their default statements.  While this is quite minor, the code flow in
wrmsr_hypervisor_regs() was sufficiently opaque that I felt it approprate to
fix.

Other improvements include:
 * not shadowing the function parameter 'idx'.
 * use of PAGE_{SHIFT,SIZE} instead of opencoded numbers.
 * a more descriptive error message for attempting to write invalid indicies
   for hypercall pages.

There is no behavioural change as a result.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>