LUInstanceCreate: factor out disk checks
LUInstanceCreate: factor out disk checks

The 'CheckArguments' method of 'LUInstanceCreate' is way
too long and should be broken up in smaller pieces. This
patch starts with that and factors out the disk checks.
No functional changes whatsoever.

Signed-off-by: Helga Velroyen <2d0126b1ec6f9ec5cbec4a7012aa14faeaef10f5@google.com>
Reviewed-by: Michele Tartara <05f7077a2c6cbbdc4af4ba42e3ef89a0df846179@google.com>

onponomarev/ganeti,dimara/ganeti,ganeti-github-testing/ganeti-test-1,ganeti/ganeti,andir/ganeti,andir/ganeti,andir/ganeti,ganeti/ganeti,apyrgio/snf-ganeti,mbakke/ganeti,bitemyapp/ganeti,yiannist/ganeti,yiannist/ganeti,mbakke/ganeti,apyrgio/ganeti,onponomarev/ganeti,leshchevds/ganeti,dimara/ganeti,apyrgio/ganeti,mbakke/ganeti,yiannist/ganeti,grnet/snf-ganeti,leshchevds/ganeti,grnet/snf-ganeti,bitemyapp/ganeti,ganeti/ganeti,leshchevds/ganeti,ganeti-github-testing/ganeti-test-1,apyrgio/snf-ganeti