taskdata: refactor get_providermap

taskdata: refactor get_providermap

Added optional parameter 'prefix' to filter out names that
don't start with specified prefix. Changed existing call
of get_providermap according to changed API.

Optimized the code: got rid of extra loop and temporary
list variable virts.

Signed-off-by: Ed Bartosh <ed.bartosh@linux.intel.com>
Signed-off-by: Elliot Smith <elliot.smith@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>