i386/msr: Extract and improve MSR support detection code

i386/msr: Extract and improve MSR support detection code

Currently rdmsr and wrmsr commands have own MSR support detection code.
This code is the same. So, it is duplicated. Additionally, this code
cannot be reused by others. Hence, extract this code to a function and
make it public. By the way, improve a code a bit.

Additionally, use GRUB_ERR_BAD_DEVICE instead of GRUB_ERR_BUG to signal
an error because errors encountered by this new routine are not bugs.

Signed-off-by: Daniel Kiper <daniel.kiper@oracle.com>
Signed-off-by: Sergii Dmytruk <sergii.dmytruk@3mdeb.com>
Reviewed-by: Ross Philipson <ross.philipson@oracle.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>