vl: merge -accel processing into configure_accelerators

vl: merge -accel processing into configure_accelerators

The next step is to move the parsing of "-machine accel=..." into vl.c,
unifying it with the configure_accelerators() function that has just
been introduced.  This way, we will be able to desugar it into multiple
"-accel" options, without polluting accel/accel.c.

The CONFIG_TCG and CONFIG_KVM symbols are not available in vl.c, but
we can use accel_find instead to find their value at runtime.  Once we
know that the binary has one of TCG or KVM, the default accelerator
can be expressed simply as "tcg:kvm", because TCG never fails to initialize.

Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>